package: use bs58check

This pull request removes `src/base58check.js`, and like `bs58`, uses the package [`bs58check`](https://www.npmjs.org/package/bs58check).
This will allow for greater software re-use where users don't necessarily want to include the entire `bitcoinjs-lib` package.

It also further illustrates why https://github.com/bitcoinjs/bitcoinjs-lib/pull/231 `compile-bundle` may be necessary in the near future.