Parse labeled tuple patterns and types

[rtjoa]

Summary

Parses labeled tuple patterns using the following temporary syntax. Note the use of punning, punning + annotations, and mixed labeled and non-labeled arguments. Labels are separated from their patterns with =, components are separated with ; (desired syntax: : and ,, respectively).

let ~~(~x=x0; ~s; ~(y:int); _) = ~~(~x: 1, ~s: "a", ~y: 2, "ignore me")

Parses labeled tuple types using the following temporary syntax:

val mytup : ~~(x:int * y:string * string)

Testing

Added testcases to ocaml/testsuite/tests/typing-labeled-tuples/labeledtuples.ml.

To test -dsource, added a testcase to ocaml/testsuite/tests/typing-labeled-tuples/labeledtuples_dsource.ml.

Added CR: labeled tuples throughout areas that will eventually need more thorough testing.

[ccasin]

This looks good.

I have some minor comments, and I think this PR should probably also update core_type_desc, since you have working examples with labeled tuple type expressions - see the comment in typetexp.ml

[ccasin]

This all looks good now, thanks! I pushed a commit that just regenerates the parser, since the last commit changed the mly, and I'll merge once the tests pass.