Add simple SDMF capability representation

See merge request !6

Add a Keys module with types and derivation functions

Closes #6

See merge request !5

* 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.

Standardize on cryptonite

See merge request !4

The test suite fails, though.

This is probably better than pulling in three different crypto libraries for
each of the difference pieces we need.  It's certainly simpler to figure out
how to make the pieces play nicely together since we avoid having multiple
kinds of AES128 key defined, multiple types named IV and PublicKey, and even
multiple implementations of the same module.

need to move the key stuff to a new branch and finish it on its own probably

k and N not confusing variable names at all...

Lots of fields left incorrectly populated but at least the erasure encoding is
working so we can probably read (without verification) real shares now.

Add a Binary instance to deserialize from and serialize to the canonical byte representation

Closes #4

See merge request !3

Specifically, this will build our new `make-keypairs` executable.

Add a representation of the share data

See merge request !2

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

Just a round-trip-through Binary test for now

* We can always interpret the IV so represent it using the IV type instead of
  as bytes.

* We cannot interpret the private key without its decryption key so represent
  its encrypted form as bytes.