This change completes commit 9dc0617e and makes all the tests pass.
It corresponds with the ongoing langauge reference documentation effort.
Fixes#781.
This fixes the following issues:
- on Python 3.10 and earlier, storing to free variables is now handled
correctly
- on Python 3.11, `_varname_from_oparg` is now used, fixing problems
with cell variables that are also arguments
- on all supported versions, EXTENDED_ARG is now parsed, ensuring proper
handling for long functions
Fixes#792.