Cardano Updates

Extracting `SecretKey` construction outside, makes resource allocation
in `bracket` much cheaper, which is absolutely what we want for proper async
exception handling

We also need to make sure that tag is fully set to zero, since it
doesn't llook like the full overwrite is guaranteed.

* `ekey` -> `skey`
* `ENCRYPTED_KEY_SIZE` -> `UNENCRYPTED_KEY_SIZE`

It was incorrectly named, since secret key in that structure is not encrypted

There was need to make a copy of the full `KeyMaterial` into MLocked
memory, which is scarce, just to validate public key against a secret key

There was a totally unnecessary allocation of key material legacy struct

Also introduce a more descriptive `mkEncryptedKey` function that
deprecates `encryptedKey`

Also deprecate `byteStringToByteArray` in its favor