FFI tutorial: explicitly document how to define a function callable from C

[pnkfelix]

I believe one is able to define C -> Rust function (Rust code callable from C) like this:

extern "C" fn foo(x: ::std::libc::c_int) -> ::std::libc::c_int { 2 * x }

but the construction above is at best only hinted at in the current tutorial-ffi.md

[pnkfelix]

part of #11755

[steveklabnik]

http://doc.rust-lang.org/guide-ffi.html#callbacks-from-c-code-to-rust-functions ?

[alexcrichton]

That covers a bit of it, but there's no discussion of #[no_mangle] which is often quite useful.

[thestinger]

It also doesn't mention the necessity to catch failure and either translate it into an error for the C caller or abort if that's not possible. It's undefined behaviour to unwind from Rust into C code.