Split current "FFI" chapter, cover more topics

[japaric]

The current chapter needs to be split:

• #[link] attribute, ffi functions are unsafe
• safe wrappers

These topics also need to be covered:

• repr(C) attribute for structs
• callbacks
• exposing rust functions extern fn to C (does this belong here?)

[bbrodriges]

It would be great if you could add an example to FFI chapter based of this Stackoverflow question http://stackoverflow.com/questions/26117197/create-interface-to-c-function-pointers-in-rust

[steveklabnik]

I'm going to close this in favor of #643