Clean up structure and expose ContextBase

[bifurcation]

This PR makes a few restructurings to clean up the structure of the library:

• The class hierarchy is now stricter. Instead of SFrame -> {Context, MLSContext}, we have:
  • ContextBase - Core protect/unprotect functionality, irrespective of header encoding
    • ContextBase::KeyState loses its counter member and becomes KeyAndSalt
  • Context: protected ContextBase - Also tracks counters, encodes/decodes header
  • MLSContext: protected Context - Also derives per-sender keys from MLS secrets
    • MLSContext::EpochKeys no longer stores keys/counters
    • Instead, we use the storage in ContextBase and Context
• The Header class is restructured so that it owns an encoded representation of the header
  • When constructing from kid+ctr, the values are encoded
  • When constructing from an encoded value, the encoded form is copied in and values are parsed