Adopt rustls-ffi arc_castable!, box_castable!, ref_castable! macros

[cpu]

See rustls/rustls-ffi#404

It would help with the eventual move towards shared FFI helpers if this repo also adopted the Castable helper macros recently added in rustls-ffi.

Filing this as an issue instead of a PR making the switch to avoid unnecessary conflicts with in-progress work. We can do this at some future point.

[cpu]

I randomly thought about this and started looking at it. Based on how this codebase evolved I think the macros are of less use here.

Unlike in rustls-ffi we're not creating new opaque structs for each Castable at the site of implementing that trait and instead we're following a pattern of defining an unexported rust type in a sub module and then exporting a SCREAMING_SNAKE_CASE type alias in entry.rs. That means less boilerplate to motivate the macro.

Similarly, unlike in rustls-ffi we sometimes want the type alias to be for one rust type, but the Castable::RustType to be that type wrapped in NotThreadSafe. Handling that in the macros complicates them, and I think obscures the key detail of the castable being NotThreadSafe or not.

Overall: a nice idea but one I think we should pass on for now.