It used to be that the caller would supply a buffer to write the output
to. This created an anti-pattern in multiple places where the caller
would allocate a `ByteBuffer` and then use `.bytes()` to provide it to
the `PKSystem` method. Then the callee would resize the output buffer
and reassign it, but because the resize was on `Bytes` and not on
`ByteBuffer`, the caller using the latter would cause a bug.
Additionally, in pretty much all cases the buffer was pre-allocated
shortly before.
- Removed the constructor taking a (n, d, e) tuple and moved
it to `RSAPrivateKey`
- Removed default constructor with key generation because it was always
misused and the default key size is quite small
- Added utility constructors to accept a key pair, public key, private
key or both
- Made constructor parameters const
- Updated test to use generated random keys where possible
