Send serialized versions of TT terms in pprint

This is the basis for commands that manipulate previously-output terms.
Add commands for working with serialised terms

In particular, IDEs can now request the implicit and explicit versions
of previously-output terms. They can also request normalisation.
Send original terms in error messages

This allows IDE users to normalise terms or show implicits in errors.
Annotate displayed IDE metavars with original terms
Maintain pretty-printer bound vars

This means serialized terms, when normalized, will have correct colors
for bound variables.
Preserve bound var info across implicitness changes
Merge pull request #1315 from david-christiansen/serialized-terms

Serialize original TT terms, and annotate pretty-printer output