Add support for new local syntax

[tdelvecchio-jsc]

This adds support for modes and modalities in the following positions:

• Let bindings:
  let x @ mode = y
let x : typ @@ mode = y
• Expressions:
  let _ = (x : typ @@ mode)
• Arrow params:
  type t = typ @ mode -> lbl:typ @ mode -> typ @ mode
• Modalities on record fields:
  type t = { x : typ @@ modality }
• Modalities on constructor arguments:
  type t = A of typ @@ modality * typ @@ modality
• Value descriptions:
  val x : typ @@ mode
• Patterns:
  let ((x @ mode), (y @ mode)) = z
• Let-bound functions:
  let (f @ mode) (x @ mode) (y @ mode) = z

[tdelvecchio-jsc]

Consider refactoring.

[tdelvecchio-jsc]

This desugaring no longer happens; the constraint is now encoded in pvb_constraint.

[tdelvecchio-jsc]

This warning being commented out was the pain source of a bug for a couple hours. I don't wee why it's commented out when there's only like 2 places below that needed to be changed to make the warning pass.

[tdelvecchio-jsc]

Tested by smashing tree to add mode annotations everywhere possible in jane, and running ocamlformat on entire tree. Obviously could have missed adding annotations in some places, and the annotations are added in a relatively uniform way, but this did successfully reveal a few bugs.

[tdelvecchio-jsc]

Currently, CI is failing on some tests unrelated to this change. I'm looking into this, but I don't think it should block review.

Update: Resolved.

[tdelvecchio-jsc]

Consider whether PR 2706 to flambda affects this PR.

[freemagma]

Consider whether PR 2706 to flambda affects this PR.

It shouldn't. The parser-extended changes I made here were based on a compiler branch that was a superset of that PR