Split prettification from proof mode reduction

See merge request FP/iris-coq!170

make `wp_alloc l as "?"` behave as expected

See merge request FP/iris-coq!169

pm_reduce just reduces away proof mode terms using cbv; pm_prettify just prettifies user-visible connectors using cbn.
Most uses of pm_default are converted to default to keep the desired reduction behavior.

List literals reduce with a spurious `emp` at the end, which is not pretty.

Telescope infrastructure

See merge request FP/iris-coq!161

With a pretty proof by Robbert

Just to cover more cases, hopefully.

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