Make crypto.rs into a module and split into files. (#3276)

## Motivation

Soon I'll be adding a new signature scheme (secp256k1) and I thought
`crypto.rs` started being too big and too generic. It'll be more
managable to have different signature schemes and hashing in its
respective files.

## Proposal

Turn `crypto.rs` file into a `crypto` module and extract code to
separate files for:
- `CryptoHash` as `hash.rs`
- `Signature`as `signature.rs`
- `KeyPair` and `PublicKey` as `keypair.rs`

## Test Plan

N/A

## Release Plan

- Nothing to do

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)