Add type constraint to binding_op

[gpetiot]

Contributing to #2401

The main positive is removing Sugar.type_cstr (all this code is inconvenient for the polynewtypes shenanigans) by doing this work in the parser.

A minor side effect is that we don't add useless parentheses around:

• let* ({ state; server; pool } as t) = xxx
• let+ (Ok x) = xxx

It's a fix, but not important enough to add a line in the changelog.

[gpetiot]

I think I fixed all regressions, test_branch doesn't report any diff.