Types:
- Imp -> IMP
- Object -> objc_object
- Sel -> objc_selector
- Protocol -> objc_protocol
- Class -> objc_class
- Ivar -> objc_ivar
- Method -> objc_method

New type aliases:
- id
- SEL
- Class
- Ivar
- Method
- Protocol (alias of objc_protocol, not a pointer)

Makes it possible to assign which pointers are mutable and which are immutable.

Also remove redundant (the original) function definitions

Makes it possible for downstream users to customize the linking to libobjc, see https://doc.rust-lang.org/cargo/reference/build-scripts.html#overriding-build-scripts

This is the lowest version we'll target because they added support for the `objc_msgSend` family of functions which we might want to use (instead of the current `objc_msg_lookup`). Alternatively we could have chosen v1.6, but they're both from 2013 so it's kinda moot at this point. v1.5 is missing lots of functionality.

Commit gnustep/libobjc2@50a797f

This can be used to see the main differences in the objc API from v1.7 to v2.1; in essence, the API we'll be using haven't actually changed!

Commit gnustep/libobjc2@282486d

Also add objc_retainAutoreleaseReturnValue that I missed in 052bcd6

Probably all of the underlying APIs can handle NULL values, so not having this possibility is restricting.

And most of them can sometimes _return_ a NULL value, so not handling that is unsound!

It's unclear how much these are part of the ABI stability, and whether they're actually useful