This supersedes #1858 and somehow addresses parts of MPR#7812. cc @whitequark

A new function is added:

val chop_suffix_opt: suffix:string -> string -> string option
(** [chop_suffix_opt ~suffix filename] removes the suffix from
    the [filename] if possible, or returns [None] if the
    filename does not end with the suffix.

The goal is to deprecate Filename.chop_suffix eventually. That function is dangerous: it only removes the final characters from the filename, without actually checking the suffix matches; and it raises only when the filename is "too short", which is rare enough to have the bugs fly under the radar for a long time.

Following the "modern" style in the stdlib, the new function returns an option instead of raising an exception in case of mismatch. This could avoid bugs such as MPR#1858.

The function uses a labelled argument to avoid the possible confusion between the two arguments. check_suffix/chop_suffix are rare cases where I always need to look up the documentation to know the ordering of arguments.

This PR does not rewrite the uses of chop_suffix in the core distribution to use the new function. This can be done as a follow-up PR.

Also, this PR does not mark Filename.chop_suffix as being deprecated; in the latest caml-devel meeting, it was mentioned that having one full major version of OCaml with the new functions before deprecating the "old" ones is considered to be the polite behavior towards the community.