Extract FFI API into separate crate(s)

[sneurlax]

monero-rust should use Rust structs etc.

so as not to undo certain precautions taken by monero-serai/monero-wallet for zeroizing etc

[sneurlax]

Current work on monero-rust integrate FFI orientated choices which do in fact undo certain precaution of monero-oxide etc to do things well and by best practices

We should still move those things out into eg. monero-ffi etc, or perhaps monero-c as an analog and drop-in replacement for monero_c in terms of eg. cs_monero

with a monero-rs responsible similarly for providing a monero-java, monero-cpp, etc style API