Changes

matching: use constructor descriptions instead of tags as matching keys

This loses no information (descriptions contain the tag), but it will
make it easier to obtain the descriptions inside `combine_constructor`
without doing a dynamic check on the patterns. This will in turn help
simplify the interaction with `Parmatch.complete_constrs`.

Note: after this patch we use `Types.equal_tag` instead of `( = )` to
compare tags during pattern-matching compilation. This is better code,
and we believe that it does not change the behavior: `Types.equal_tag`
is mostly similar to a type-specialized version of `( = )`, except
that it calls `Ident.same` that just compares the stamps and ignore
the names, which (assuming well-formedness of idents) is equivalent
and slightly faster.
 (commit: d333ac8)

Matching: propagate constructor descriptions in complete_pats_constrs

This simplifies this particular interface boundary between Matching
and Parmatch.

(Suggested by Florian Angeletti)
 (commit: bf95a24)