Add FFI/C API

[mtreinish]

To enable interfacing retworkx python objects with other languages directly instead of going through python for better performance we should add a foreign function interface and publish header files.

[mtreinish]

https://github.com/eqrion/cbindgen/blob/master/docs.md has details on how to do this.

[mtreinish]

Also: https://michael-f-bryan.github.io/rust-ffi-guide/ is useful

[IvanIsCoding]

Apparently there is a Rust-to-Rust FFI crate: https://github.com/rodrimati1992/abi_stable_crates/

I haven’t read the details yet but it could be interesting