Proto/Michelson: simplify parse_comparable_data/ty

Context

Follows !4502 (merged), !4721 (merged), !4722 (merged), and !4723 (merged), i.e. simplifications after removal of comparable_ty.

Here, we use parse_data instead of parse_comparable_data, so we're sure we do parse the same way.

We also use parse_ty in parse_comparable_ty and check is_comparable on the result type, again no need to maintain 2 versions of the same thing!

I wanted to do unparse_comparable_data too (hence the 2nd commit, the beginning of an attempt) but unparse_comparable_data is used by hash_comparable_data, itself used in parse_data (because of bigmap overlays using key hashes, hein @klakplok 😉). So merging unparse_comparable_data with unparse_data would mean having almost everything being mutually recursive... maybe we'll do it another time.

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