77: Move automatic generation of Unpin implementation to proc-macro-derive r=taiki-e a=taiki-e

To generate the correct `Unpin` implementation, we need to collect the
types of the pinned fields. However, since proc-macro-attribute is
applied before cfg, we cannot be collecting field types at this timing.
So instead of generating the `Unpin` implementation here, we need to
delegate automatic generation of the `Unpin` implementation to
proc-macro-derive.

Fixes #68

Co-authored-by: Taiki Endo <te316e89@gmail.com>

109: Release 0.4.0 r=taiki-e a=taiki-e

cc #21

### Changes since the latest 0.3 release:

* **Pin projection has become a safe operation.** In the absence of other unsafe code that you write, it is impossible to cause undefined behavior. (#18)

* `#[unsafe_project]` attribute has been replaced with `#[pin_project]` attribute. (#18, #33)

* The `Unpin` argument has been removed - an `Unpin` impl is now generated by default. (#18)

* Drop impls must be specified with `#[pinned_drop]` instead of via a normal `Drop` impl. (#18, #33, #86)

* `Unpin` impls must be specified with an impl of `UnsafeUnpin`, instead of implementing the normal `Unpin` trait. (#18)

* `#[pin_project]` attribute now determines the visibility of the projection type/method is based on the original type. (#96)

* `#[pin_project]` can now be used for public type with private field types. (#53)

* `#[pin_project]` can now interoperate with `#[cfg()]`. (#77)

* Added `project_ref` method to `#[pin_project]` types. (#93)

* Added `#[project_ref]` attribute. (#93)

* Removed "project_attr" feature and always enable `#[project]` attribute. (#94)

* `#[project]` attribute can now be used for `impl` blocks. (#46)

* `#[project]` attribute can now be used for `use` statements. (#85)

* `#[project]` attribute now supports `match` expressions at the position of the initializer expression of `let` expressions. (#51)

### Changes since the 0.4.0-beta.1 release:

* Fixed an issue that caused an error when using `#[pin_project(UnsafeUnpin)]` and not providing a manual `UnsafeUnpin` implementation on a type with no generics or lifetime. (#107)


Co-authored-by: Taiki Endo <te316e89@gmail.com>

135: Move most of the processing to proc-macro-derive r=taiki-e a=taiki-e

To generate the correct `Unpin` implementation and the projection methods, we need to collect the types of the pinned fields.  However, since proc-macro-attribute is applied before `#[cfg]` and `#[cfg_attr]` on fields, we cannot be collecting field types properly at this timing. So instead of generating the `Unpin` implementation and the projection methods here, delegate their processing to proc-macro-derive.

#77 moved only the automatic generation of `Unpin` implementation to proc-macro-derive but found that it could not support `#[cfg_attr]`, so this moves more processing.

See also #68 (comment).

This also adds support for the use of `#[cfg]` on tuple structs and tuple variants.

Closes #123

Co-authored-by: Taiki Endo <te316e89@gmail.com>