PEP 558: adjustments after updating implementation

[ncoghlan]

• pdb stores __return__ and `exception`` entries on
  arbitrary frames, so that feature needs to be preserved
• Some fast locals proxy operations will implicitly update the
  underlying shared mapping on the frame
• Anyone explicitly calling LocalsToFast is going to want the
  read/write proxy, not any of the read-only options (plus it's
  hard to fit an essay in an error message)
• Remove lingering reference to the removed PyLocals_RefreshViews()

[ncoghlan]

@njsmith @markshannon The reference implementation is largely back to only needing "turn the crank" type work to flesh out the Python and C APIs - the core technical issues (including harmonisation with the 3.11 frame implementation changes) have all been resolved. There are some performance problems with the proxy due to the need to support extra keys to keep pdb happy, but consumers can mitigate those with a call to f_locals.copy() to get a true snapshot that won't redundantly refresh itself when it doesn't need to.

My current plan is to post this version to python-dev next weekend, requesting pronouncement from Nathaniel.