improve FFI layer

[chunningham]

Use a tool like flapigen to remove the need for hand-written FFI code between Rust and Java/ObjC modules

[chunningham]

we can also use napi-rs for single-module node integration (no more multi-platform multi-version optional dependancy multiplexing)

[chunningham]

#8 provides an example of flapigen usage which we can build out to the RN bindings

[chunningham]

neon will adopt the N-API soon, so the various versions for the node modules can be removed once neon has implemented that change