Implicit source positions object system support

[Enoumy]

This feature adds support for "implicit source positions" ([%src_pos] arguments) to OCaml's object system. Allowing for:

class c ~(src_pos : [%src_pos]) () = object method position = src_pos end
let position : lexing_position = (new c ())#position
[%%expect{|
class c :
  src_pos:[%src_pos] -> unit -> object method position : lexing_position end
val position : lexing_position = (* position of the callsite of the class being created! Where the [new c ()] is. *)
|}]

Testing

• New tests added to the typing-implicit-source-positions test directory. The tests follow the pattern of how implicit source positions were implemented for normal expressions and also adds some new tests just for the object system specifics.
  • Type error tests
  • Unerasable arguments
  • Shadowing tests, commutatitivity tests
  • Positional argument being erased in a class_expr (e.g. inherit CLASS_EXPR)
• Additional "drive-by" testing from the "conflict resolution" in the parent feature to sanity check that the unique_ keyword interacts nicely with the src_pos argument.

For reviewers

The approach for this feature is very similar to the one taken for normal OCaml expressions.

There were some place in typeclass.ml that are a bit code duplicate-ey between how src_pos arguments and optional arguments are erased. In places where it was trivial/localized to de-duplicated I de-duplicated, but in typeclass.ml there were a couple of places where making things use a match would've involved larger changes and I am unsure if we prefer to make smaller, self contained changes to avoid merge conflicts with upstream OCaml. Please let me know if this is reasonable/isn't.