5.5 support: Backport parser code changes #2763
Description
Backport these changes to the vendored parsers that have no effect on the syntax:
- Uniformize parsing rules for structure and signature items ocaml/ocaml#14039
- Rename cstrs to constraints to distinguish from cstrs = constructors ocaml/ocaml#14141
- Removed unused field in package_type ocaml/ocaml#14148
- Constraints on module unpacking are not ghost ocaml/ocaml#14198
- Typos, typos ocaml/ocaml#14283