Refactor Dart integration focus into monero-ffi crate

[sneurlax]

Whereas this repo (monero-rust) is currently oriented towards use by Dart, it should become a minimally-opinionated general-purpose Monero SDK in Rust. Use by Dart via FFI should be refactored out into a separate monero-ffi library.

#2 will probably also move over.

[sneurlax]

WIP, close once complete.

[sneurlax]

Will probably move towards a monero-c crate as a shortterm goal for an optional Rust-backed cs_monero

Whereas monero-ffi may actually become focused on wrapping C++ from monero-project/monero for use in Rust programs or will otherwise provide an opinionated API