Logically atomic triples: Notation, tactics, small example

See merge request FP/iris-coq!163

make atomic_acc typeclasses-opaque

Use telescopes for atomic accessors, updates and triples; improve mask handling; add notation for all of them

Generalized proofmode

Closes #91, #198, #193, #190, #187, #182, #156, #177, #176, #147, #146, and #154

See merge request FP/iris-coq!66

See https://github.com/coq/coq/issues/8003

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.