Refactor crypto code into standalone lib

[lrettig]

We need a @spacemesh/crypto lib that:

• allows generation of new mnemonics, keypairs, and accounts
• reads/writes wallet files
• contains the Wallet data structures and methods
• can validate all of these things

This code all lives in this repository today and can be factored out into a separate library. This will be broadly useful across the Spacemesh ecosystem.

The ed25519-related code, including the wasm library, should probably be moved there too.

[lrettig]

This codebase is quite clean and well organized. It also uses Wasm under the hood as a global standard. Please have a look: https://github.com/polkadot-js/common/tree/master/packages/keyring

API: https://polkadot.js.org/docs/api/start/keyring Docs: https://polkadot.js.org/docs/keyring