Merge branch '548-require-a-label-for-hashing' into 'master'

Require a label for hashing

* Add a string label argument to `CryptoComponent#hash()`
* Convert `DoubleDigest` from implementing `MessageDigest`
  to implementing `org.spongycastle.crypto.Digest`
  (we need to keep `DoubleDigest` for `FortunaGenerator`)
* Convert all other uses of `MessageDigest` to `CryptoComponent#hash()`
* Remove `CryptoComponent#getMessageDigest()`, `MessageDigest` and `DigestWrapper`

Closes #548

See merge request !442