Symmetric crypto: add the ability to reset the state

The Intel folks noticed that in order to encrypt/decrypt a new message, a new symmetric state had to be created.

However, with AES-GCM, setting up a new key is slow, because it requires doing some precomputations.

If the same key is used multiple times, we could reuse these precomputations for a slight speedup.

One way to address this would be to introduce a symmetric_state_reset(<state handle>, <key handle>) hostcall.

If the key handle is the same as the previous one, it doesn't do anything except clear the nonce (or create a new one if it can be randomized).
If the key is different, it's equivalent to creating a new state, but without creating a new handle.

If the key doesn't change, this allows implementations to reuse the precomputations. This is optional (just a performance enhancement) and only for AES-GCM, or possibly future ciphers that have a slow key setup.

What do you think @sonder-joker ?

Start by reviewing the proposed symmetric_state_reset(, ) hostcall and the behavior described for reused and changed keys. No files or tests are named; the issue needs a settled API decision and clear completion criteria before implementation can begin.