Implement generic Fujisaki-Okamoto transform

(This issue has been imported from the Gitlab repository because it seems to not have been addressed yet)

Original Text (Issue 65)

As discussed with Gennadij just now, we can implement Fujisaki-Okamoto ("fo") transform in a blackbox way.

Some thoughts:

Some of this is already implemented somewhat generically (according to Fabian) in the .fo-package schemes
We need to give a warning in Javadoc: applying fo on schemes other than standard encryption does not necessarily result in anything secure (Gennadij said he may want to prove that for predicate encryption, though, which would immediately handle ABE, etc.)
We should probably add a new interface that fo-compatible schemes can implement
- Ensures only schemes that are secure afterwards can be run through the transform
- Adds methods to encrypt with given randomness (or random group elements as additional input)
The fo class itself should be parameterized with
- a symmetric and a (ABE/IBE/vanilla/...) asymmetric encryption scheme (with interface above)
- a hash function to derive randomness for the asymmetric scheme
- a key derivation function for the symmetric scheme
Implementations should probably extend the fo class. We can again do this generically for every kind of scheme (ABE/IBE/vanilla encryption...)
Open problem: how to fo-encrypt/decrypt in JCA? If the asymmetric encryption is at the end of the ciphertext stream, decryption is impossible without looking at the end of the stream first. The other way around is impossible for encryption. Overall, this stream model is difficult. Buffering is a simple fix, but obviously not incredibly elegant.

After this is done, there will be no need for the individual .fo packages anymore.