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make pm_maybe_wand a BI connective; reduce BI connectives and option combinators in the proofmode with cbn

See the comments in the code for why that makes sense

This restricts CAS to only be able to compare literals with literals, NONEV with NONEV and NONEV with SOMEV for a literal.

All those for `big_sepL` that hold.

We already do the same for the goal, this avoids some scope
delimiters being displayed.

* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics
* use reduction-controlled `pm_default` for proofmode accessors