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

Allow comparing even more values in CAS by adding non-determinism

See merge request FP/iris-coq!167

Thanks to Siddharth for pointing this out

See the comments in the code for why that makes sense

Make CAS slightly more realistic

See merge request FP/iris-coq!165

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