* Add direct tests for signature key serialization/deserialization
* Fix the RSA Private Key serializer to use PKCS8 like Tahoe-LAFS expects.
* Fix the key generator to emit keys in this format.
* Fix the key generator to generate 2048 bit keys.
* Fix the derivation functions to make the key values internally consistent.

The previous keys were not serialized as x509 Private-Key Info structures
which is the format Tahoe-LAFS expects us to operate on.

Read Tahoe-LAFS-generated bytes, decode into a Share, re-encoded to the same
bytes.

Just a round-trip-through Binary test for now