Learn unsafe Rust: Provide initial content for the chapter "ABI and FFI"

[darkwisebear]

Todo

Provide initial content for the ABI and FFI chapter of the Learn unsafe Rust book.

Proposed content

• Explain ABI and what "C ABI/FFI" actually means
  • What aspects of the ABI are defined by C and which aren't
  • How compilers and compiler settings influence the ABI
  • What aspects are influenced by the ABI
• Explain unsafety categories in FFI
  • Mismatching calling conventions / padding (aka ABI)
  • Mismatching function signatures
  • Mismatching type declarations
  • Unsafe actions beyond the FFI boundary
  • Moveability/self-reference, especially with C++ move constructors
  • Ownership
  • C++ topics
• Mitigations for FFI unsafety
  • Existing FFI tooling (bindgen, cbindgen, cxx)
  • Checks that can be done on FFI and Rust side
  • cRABI (?)