Make TypeFoldable derivable

[Aaron1011]

Currently, the (internal) TypeFoldable trait must be implemented via the fairly verbose EnumTypeFoldableImpl! and BraceStructTypeFoldableImpl! macros. Adding a derive(TypeFoldable) proc-macro to librustc_macros would allow us to delete a large amount of code, and would drastically simplify adding new TypeFoldable implementations.

[cjgillot]

Hello,
I would like to do that.
Do you mind?

[Aaron1011]

@cjgillot: Go ahead :)

[eddyb]

Technically a subset of / related to #36588 - although I think I went a bit crazy with the ideas there, and just regular derives for a lot of traits is fine.