More examples of UB and FFI footguns

[Michael-F-Bryan]

The last chapter of the book shows potential FFI footguns and UB. This is a tracking issue for brainstorming more examples which could be added to the guide.

Existing Problems:

• [ ] Linker Errors and Name Mangling
• [ ] Unwinding
• [ ] CString.as_ptr()
• [ ] Enum From Integer is UB
• [ ] Allocator Mismatch
• [ ] Rust Enums and Tagged Unions

[fabricedesre]

I filed https://github.com/Michael-F-Bryan/rust-ffi-guide/issues/53