Typecheck labeled tuple terms

[rtjoa]

Summary

• Typecheck labeled tuple terms, e.g. ~~(~x: 1, "hi") as x: int * string
• Ensures tuple types match when unifying
• Implementation: adds a string option * to the element types of Texp_tuple's and TTuples's lists

Testing

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

Printtyp.raw_type_expr was tested via the debugger.

The type of the value ~~(~lbl:5, "hi") prints as:

Ttuple
        [label="lbl" {id=114;level=100000000;scope=0;desc=Tconstr(int,[],[])};
         {id=115;level=100000000;scope=0;desc=Tconstr(string,[],[])}]};

[ccasin]

This looks good! I have a bunch of minor suggestions, but didn't spot any big issues.