PEP 558: Update PEP for implementation changes and PEP 667

[ncoghlan]

• address remaining review comments from the July threads
• Rationale section renamed to Motivation
• Design Discussion section renamed to Rationale and Design Discussion
• kind enum is guaranteed to be at least 32 bits
• fast refs mapping is stored on the underlying frame
• delay initial cache refresh for each proxy instance to the
  first operation that needs it
• be specific about which operations always update the cache, and which
  update it if it hasn't been updated by this proxy instance
• eliminate more mentions of the old "dynamic snapshot" terminology
• add new rationale/discussion section covering PEP 667 (including
  how the PEP 558 implementation could be turned into a PEP 667
  implementation if desired)
• make it clearer that proxy instances are ephemeral (lots of
  stale phrasing with "the" dating from when they stored on the frame)