analogue of overload_info_for for types

[xrchz]

Finding out what a string might be printed/parsed as is useful for both terms and types. For terms, we have Parse.overload_info_for. For both there are various functions of the form pp_blah_without_overloads or pp_blah_without_abbrevs, but they're all pretty hard to use because you have to figure out how to deal with HOLPP and your backend.

I think this was more usable before af84d14 but still not ideal. That commit closed #13 and promised a new issue to fix the usability for terms, which appears to have been fixed by 22ae0c9 anyway. But for types and type abbreviations I think we're still missing a nice API.

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