Auto merge of #45930 - jplatte:generics_refactoring, r=eddyb

Auto merge of #45930 - jplatte:generics_refactoring, r=eddyb

Generics refactoring (groundwork for const generics)

These changes were suggested by @eddyb.

After this change, the `Generics` contain one `Vec` of an enum for the generic parameters, rather than two separate `Vec`s for lifetime and type parameters. Type params and const params will need to be in a shared `Vec` to preserve their ordering, and moving lifetimes into the same `Vec` should simplify the code that processes `Generics`.