Blanket impl `ProjectField` for struct and union types

[joshlf]

...using STRUCT_VARIANT_ID and UNION_VARIANT_ID.

This applies once #2886 lands, which introduces that machinery. That PR provides a default impl of ProjectField:: is_projectable, and uses a PME to ensure that it's overridden for enum impls. It also implements ProjectField for all struct and union types manually in their respective derives.

We can instead provide a two blanket impls – one for structs and one for unions – and thus remove both the default impl of is_projectable and the impls emitted by derive code.

We'll want to look at the rustdoc output to see how this change affects it. In particular, we want it to be the case that, for a user-defined type, the rustdoc page for that type shows ProjectField impls for that type.