…eting

This fixes several areas where the spec and implementation of ratcheting
in the encrypted_chat example did not align:

1) The spec for evolving the state used the current epoch to define the domain
   separator, then returned a new state with the new key and the next epoch. The
   implementation instead had a bug where it incremented twice, once at the
   start and once at the end.  So the key would be derived using the next epoch
   id n, but then have an epoch id of n+1. Confusing! Change both: specify that
   the next (new) epoch is the value that is added to the domain separator.

2) The implementation adds -example to the domain separator strings while the
   spec does not. Minor issue but might as well be consistent.

3) evolveTo did not evolve states ... it first checked if the requested epoch is
   less than or equal to the current epoch, in which case it returns
   early. Otherwise it looped in this fashion

      while(requestedEpoch < currentEpoch)

   which would never occur since the function would have already have returned
   in this case. I renamed the relevant argument to desiredEpoch since I think
   that makes it easier to read/reason about what is happening.

4) Fixed CacheableSymmetricRatchetState::peekAtEpoch. It pre-incremented the
   epoch before cloning the structure, but this puts it out of sync with the
   correct epoch and so different keys would be derived. This function doesn't
   seem to be used, which is probably why this wasn't noticed. It may be better
   to remove this function entirely.