Feat/ffi

[Nuhvi]

This PR packages PubkyClient and other useful structs for npm both nodejs and web using wasm_bindgen.

Features available now in both Rust and JS:

1. Generate a Keypair
2. Signup to a local testnet homeserver
3. Signout
4. Signin
5. Examine session for a public key (confirm if user is signed in).
6. Put a file
7. Get a file

Downsides of Wasm:

1. Large module approaching ~1mb, hopefully that can be optimized later, but either way it is only relevant for the first time you open a web app, where the user is busy onboarding, frontend developers should be able to draw content without being blocked by that initial download.
2. Outside of the happy path, stack traces will probably be unhelpful.

Upside of Wasm:
Surprisingly great code sharing, especially when accepting the async paradigm.