Allow exporting key material and init of cipherstate

[aidantwoods-1p]

In order to allow implementations to store the nonce and key (e.g. for use with distributed systems), I've added an UnsafeKey method that exports the current key for storage. Combined with the CipherSuite, already exportable nonce, and this key, these can be used to reconstruct a CipherState from stored data using the new UnsafeNewCipherState method.

Noting that UnsafeKey isn't strictly necessary if it is already accessible from the Cipher, but I think this is preferable overall to needing to possibly create wrapped Cipher types which allow the key to be later exportable.

The Unsafe keyword is mean to indicate that these functions should be avoided at a glance, unless the caller accepts responsibility for manually managing exported state outside of application memory.

[titanous]

This looks good, just minor nits in the docs.

[aidantwoods-1p]

Nits now resolved 🙂

[complexspaces]

Thank you for taking a look at this so quick for us 🎉