add name hint metadata to terms for pretty-printing

[danmatichuk]

partially addresses #3073

Attaches some provenance information to subterms that may be used when inventing variable names for memoization.

Currently this data exists as a field in Term, which is a bit wonky because it violates the invariant that terms with the same index must be equal. However, for most purposes this doesn't matter, it (should) only be relevant for printing. The current downside is that term caching can cause some provided name hints to be ignored (instead using the name hints that were present when the cache entry was made).

Update: see comment

[danmatichuk]

I've re-implemented this in order to make the term metadata a formal TermF constructor. This required some changes to a number of primitives, so that they both ignore the metadata wrappers, and preserve metadata wherever appropriate.