Refactor to separate wasm bindings from rust crate

[michaelx11]

Goal

This PR follows the structure of other example Rust crossplatform libraries I've seen (link)

The structure is:

• Move core implementation in Rust into a separate crate, unaware of any platform bindings
• Move bindings into their own subdirectories and crates

In this case it manifests as:

• New xmtp-keystore crate (open to renaming ideas)
• Move existing repo contents into bindings/wasm subdirectory

Changes

• Restructuring as mentioned above
• In xmtp-keystore, we build with ethers-rs just to show we can, and generate a mnemonic to show we can exercise code
• In bindings/wasm we add a dependency on xmtp-keystore, modify the API, and add tests to exercise generateMnemonic

Testing

• npm run prepublishOnly now shows two tests running successfully.

 ✓ tests/wasmpkg.test.ts (2)

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  12:41:58
   Duration  508ms (transform 208ms, setup 0ms, collect 10ms, tests 4ms)