Initial Proof of concept for FFI encoder

I am using this library in a FFI scenario. And while using the derive I can encode/decode types properly, I find in a few scenarios I need "AD-HOC" encoding decoding.

This is a simple proof of concept that encodes a byte array via a FFI interface.

I originally thought it would be best as a separate crate (IE minicbor-sys) however, I had to add the encoder/decoder struct with some ffi attributes. I use the safe-ffi crate to support slices across the FFI boundries.

Ideally, other additions from here would only go into the minicbor-sys folder. But I thought I see what your thoughts were on FFI support before going further.

Signed-off-by: thomas thomas.chiantia@gmail.com