Consider making objc no_std compatible

[SSheldon]

This crate is mainly just a wrapper around the objc runtime dylib, so it shouldn't need much else. If we can make it no_std then it might be usable in more places, perhaps even in the rust compiler itself.

The current std dependencies that aren't part of libcore are:

• Nul-terminating &str via CString in the runtime module and the declare module
• Reading the length of a C string via CStr in the runtime module and for Encoding
• Using a String in Encoding to store dynamically-sized, runtime defined type encodings
• Allocating a buffer via String when declaring methods to represent their type encodings in the declare module
• Using a String in the representation of MessageError
• Implementing the Error trait for MessageError
• Using the std::os::raw types instead of the libc types in the runtime module and for implementing the Encode trait

[SSheldon]

Current thoughts on how this could be possible:

• [ ] #54: create a new string abstraction so nul-terminated strings can be passed to the runtime without us having to modify them and allocate new memory
• [x] #53: overhaul encoding, maybe make it generic so that it can sometimes be declared without any allocations
• [ ] break the declare module into a separate crate
• [ ] convert MessageError into an enum rather than backing it with a String and remove the Error conformance (rust-lang/rust#32500)
• [ ] switch back to using libc instead of std::os::raw 😕