As mentioned in ocaml-ppx#1229

Remove parentheses in:
```diff
 ;;
-(() [@attr])
+() [@attr]

 ;;
-[%ext (() [@attr])]
+[%ext () [@attr]]
```

As mentioned in ocaml-ppx#1229

Remove parentheses in:
```diff
 ;;
-(() [@attr])
+() [@attr]

 ;;
-[%ext (() [@attr])]
+[%ext () [@attr]]
```

* Fix missing parentheses around let open

Fixes #1226

Parentheses around  were removed, changing the AST in some cases.

This adds a regression, it is consistent with other expressions and
should be discussed separately:
```
-  [%foo M.(()) [@foo]] ;
+  [%foo (M.(()) [@foo])] ;
```

* Remove unecessary parentheses in extensions

As mentioned in #1229

Remove parentheses in:
```diff
 ;;
-(() [@attr])
+() [@attr]

 ;;
-[%ext (() [@attr])]
+[%ext () [@attr]]
```

CHANGES:

#### New features

  + Add an option `--format-invalid-files` to print unparsable parts of the input as verbatim text. This feature is still experimental. (ocaml-ppx/ocamlformat#1026) (Guillaume Petiot)

  + Support multi-indices extended indexing operators (ocaml-ppx/ocamlformat#1279, ocaml-ppx/ocamlformat#1277) (Jules Aguillon, Guillaume Petiot)
    This feature has been added in OCaml 4.10.0

  + Handle OCaml 4.10.0 AST (ocaml-ppx/ocamlformat#1276) (Guillaume Petiot)

  + Preserve functor syntax for consistency (ocaml-ppx/ocamlformat#1312) (Guillaume Petiot)
    Previously both functor syntax: `module M = functor (K : S) -> struct end` and `module M (K : S) = struct end` would be formatted as the latter, the original syntax is now preserved.

#### Changes

  + Add the option `doc-comments-val=before|after` (ocaml-ppx/ocamlformat#1012) (Jules Aguillon)
    This option set the placement of documentation comment on `val` and `external` only.
    It is set to `after` by default.

  + The default for `doc-comments` is changed from `after` to `before` (ocaml-ppx/ocamlformat#1012, ocaml-ppx/ocamlformat#1325) (Jules Aguillon)
    This affects both `conventional` (default) and `ocamlformat` profiles.

  + Some options are now deprecated:
    * `doc-comments` (ocaml-ppx/ocamlformat#1293, ocaml-ppx/ocamlformat#1012)
      This option depends on a flawed heuristic.
      It is replaced by `doc-comments-val` for `val` and `external` declarations.
      There is no equivalent to this option in the general case.
    * `escape-chars`, `escape-strings` and `extension-sugar` (ocaml-ppx/ocamlformat#1293)
      These options are rarely used and their default behavior is considered to be the right behavior.

  + Add space between `row_field` attributes and the label or arguments, to be
    consistent with the non-polymorphic case. (ocaml-ppx/ocamlformat#1299) (Craig Ferguson)

#### Bug fixes

  + Fix missing parentheses around `let open` (ocaml-ppx/ocamlformat#1229) (Jules Aguillon)
    eg. `M.f (M.(x) [@attr])` would be formatted to `M.f M.(x) [@attr]`, which would crash OCamlformat

  + Remove unecessary parentheses with attributes in some structure items:
    * extensions and eval items (ocaml-ppx/ocamlformat#1230) (Jules Aguillon)
      eg. the expression `[%ext (() [@attr])]` or the structure item `(() [@attr]) ;;`
    * `let _ = ...`  constructs (ocaml-ppx/ocamlformat#1244) (Etienne Millon)

  + Fix some bugs related to comments:
    * after a function on the rhs of an infix (ocaml-ppx/ocamlformat#1231) (Jules Aguillon)
      eg. the comment in `(x >>= fun y -> y (* A *))` would be dropped
    * in module unpack (ocaml-ppx/ocamlformat#1309) (Jules Aguillon)
      eg. in the module expression `module M = (val x : S (* A *))`

  + Fix formatting of empty signature payload `[%a:]` (ocaml-ppx/ocamlformat#1236) (Etienne Millon)

  + Fix parenthesizing when accessing field of construct application (ocaml-ppx/ocamlformat#1247) (Guillaume Petiot)

  + Fix formatting of attributes on object overrides `{< >}` (ocaml-ppx/ocamlformat#1238) (Etienne
    Millon)

  + Fix attributes on coercion (ocaml-ppx/ocamlformat#1239) (Etienne Millon)

  + Fix formatting of attributes on packed modules (ocaml-ppx/ocamlformat#1243) (Etienne Millon)

  + Fix parens around binop operations with attributes (ocaml-ppx/ocamlformat#1252, ocaml-ppx/ocamlformat#1306) (Guillaume Petiot, Craig Ferguson)

  + Remove unecessary parentheses in the argument of indexing operators (ocaml-ppx/ocamlformat#1280) (Jules Aguillon)

  + Retain attributes on various AST nodes:
    * field set expressions, e.g. `(a.x <- b) [@A]` (ocaml-ppx/ocamlformat#1284) (Craig Ferguson)
    * instance variable set expressions, e.g. `(a <- b) [@A]` (ocaml-ppx/ocamlformat#1288) (Craig Ferguson)
    * indexing operators, e.g. `(a.(b)) [@A]` (ocaml-ppx/ocamlformat#1300) (Craig Ferguson)
    * sequences, e.g. `(a; b) [@A]` (ocaml-ppx/ocamlformat#1291) (Craig Ferguson)

  + Avoid unnecessary spacing after object types inside records and polymorphic variants,
    e.g. `{foo : < .. > [@A]}` and `{ foo : < .. > }` (ocaml-ppx/ocamlformat#1296) (Craig Ferguson)

  + Fix missing parentheses around tuples with attributes. (ocaml-ppx/ocamlformat#1301) (Craig Ferguson)
    Previously, `f ((0, 0) [@A])` would be formatted to `f (0, 0) [@A]`, crashing OCamlformat.

  + Avoid emitting `>]` when an object type is contained in an extension point
    or attribute payload (ocaml-ppx/ocamlformat#1298) (Craig Ferguson)

  + Fix crash on the expression `(0).*(0)` (ocaml-ppx/ocamlformat#1304) (Jules Aguillon)
    It was formatting to `0.*(0)` which parses as an other expression.

  + Preserve empty doc-comments syntax. (ocaml-ppx/ocamlformat#1311) (Guillaume Petiot)
    Previously `(**)` would be formatted to `(***)`.

  + Do not crash when a comment contains just a newline (ocaml-ppx/ocamlformat#1290) (Etienne Millon)

  + Handle lazy patterns as arguments to `class` (ocaml-ppx/ocamlformat#1289) (Etienne Millon)

  + Preserve cinaps comments containing unparsable code (ocaml-ppx/ocamlformat#1303) (Jules Aguillon)
    Previously, OCamlformat would fallback to the "wrapping" logic, making the comment
    unreadable and crashing in some cases.

  + Fix normalization of attributes, fixing the docstrings in attributes (ocaml-ppx/ocamlformat#1314) (Guillaume Petiot)

  + Add missing parentheses around OR-patterns with attributes (ocaml-ppx/ocamlformat#1317) (Guillaume Petiot)

  + Fix spacing inside parens for symbols when the spacing was handled by the englobing exp (ocaml-ppx/ocamlformat#1316) (Guillaume Petiot)

  + Fix invalid (unparsable) docstrings (ocaml-ppx/ocamlformat#1315) (Guillaume Petiot)
    When parsing a comment raises an error in odoc, it is printed as-is.

  + Fix parenthesizing of optional arguments rebound to non-variables, e.g. `let
    f ?a:(A) = ()` rather than the unparsable `let f ?a:A = ()` (ocaml-ppx/ocamlformat#1305) (Craig Ferguson)