Clean up PublicKey representation

[infinisil]

Does a cleanup of public key handling, notably:

• Cleanly puts all COSE_Key constants from the COSE registry into its own module
• Cleanly implements the COSE_Key decoding in its own module
• Fixes RSA keys having wrong key sizes. Previously the RSA key size was derived from the hash algorithm.
  • E.g. since SHA-256 has a size of 256, this is the size of the RSA key. But this only somewhat worked coincidentally, because the SHA-256 size is 256 bits, whereas RSA key size was passed as bytes, leading to a key size of 2048 bits, which just so happens to be the minimum key size you should have anyways.
  • Instead, the public key size can be directly derived from its serialized n parameter.
  • This allows removing a bunch of constants from the code
  • In addition, the tests now always use 2048 bit keys, which makes them run much faster, since previously they were generating RSA keys with 4096 bits (because SHA-512 has size 512 bits, which turned to 512 bytes key size = 4096 bits)
• The number of conversion/helper functions was reduced to a minimum and mostly total ones
• Slightly improves COSE_Key conformance by making sure that leading zero-octets in ECDSA keys are preserved

[infinisil]

Still to do is cleaning up the tests