Create a core-graphics-types crate to avoid having to update so many crates when core-graphics makes breaking changes

[jdm]

core-text and cocoa depend on core-graphics because they use types from it. If we moved those types into a different crate, it would be easier to make breaking changes to core-graphics without it rippling downstream in so many places.

[jrmuizel]

See rust-lang/rust-semverver#63 for a tool that help discover the needed types.