Refactor ppaux out of existence.

[eddyb]

A long-time coming, this PR reorganizes and rewrites the pretty-printing architecture of rustc, specifically the parts that involve the typesystem (which used to be in rustc::util::ppaux).

Note: these commits used to be in #57967 before being split off.

The new API (i.e. the Printer and PrettyPrint traits) is in rustc::ty::print.

Design points, roughly:

• using associated types in Printer to allow building e.g. an AST, not just printing as a side-effect
• several overloading points for implementers of PrettyPrinter, e.g. how <...> is printed
• for fmt::Display impls, the value to print is lifted to the ty::tls tcx, and everything after that stays within the ty::print API, which requires 'tcx to match between values and the printer's tcx, without going through fmt::Display again

Most of the behavior is unchanged, except for a few details, which should be clear from the test changes.

r? @nikomatsakis

Fixes #55464