Add traits w/ auto-deriving for soundly serializing/inspecting/transforming rustc types. #36588

Currently, we're limited to the general-purpose Eq, Ord, Hash, Encodable and Decodable traits, even though the compiler often has more unconventional desires, involving contexts and custom behaviors.

There are several special-purpose traits already in the compiler, such as AST/HIR Folder/Visitor and the two-in-one TypeFoldable (which also does visiting).

Also, #36551 adds a specialization mechanism in the bundled rustc_serialize to work around the fact that Encodable and Decodable have their methods be generic over the encoder/decoder instead of the trait, but this abuses the yet-unclosed lifetime soundness hole in specialization (#31844):

impl<'a, 'tcx> SpecializedDecoder<Ty<'tcx>> for DecodeContext<'a, 'tcx> {...}

That impl requires that the 'tcx of the decoded Ty match the 'tcx of the decoder, but this cannot be enforced through specialization as the lifetime relationships do not exist before monomorphization.

A better solution would involve custom traits that can dispatch over the decoder w/o specialization, i.e.:

impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Ty<'tcx> {...}

To make all of this ergonomic, auto-deriving is necessary, and I believe macros 1.1 (perhaps helped by a permanent switch to rustbuild) can make it happen.

Last but not least, it may be possible to generalize encoding/decoding and visiting/folding/relating into a single (or a pair of) abstraction(s), but I'd have to leave that to someone who understands tree folding, transducers, and perhaps Haskell lenses, although there's no guarantee they map nicely to Rust.

cc @rust-lang/compiler