Drew's core-foundation bindings for Rust

Provides Rust bindings for Apple Core Foundation. This crate may be compared to

Similar to my other macOS libraries, this provides an elegant, performant, and largely safe wrapper for macOS low-level libraries.

Design notes:

We use opaque types to model the underlying CFType (like __CFString, etc.) Then the equivalent to CFStringRef is &CFString, a pointer type.
Some optimizations are not yet implemented. Among them, optimizations for static-time strings, inner pointers, zero-copy, etc.

The following types are at least partially implemented. Usually they contain "common APIs" or "the ones I use".