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

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

* 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

New atomic updates: defined as a fixed point with existential quantifier; intro lemma using class of Laterable assertions

There's a very low risk of these conflicting with Coq's standard library

New IntoAcc typeclass to decouple creating and elliminating accessors; ElimInv supports both with and without Hclose

Extend ElimModal with Boolean flags to specify whether it operates on the persistent/spatial context.

This is backwards-compatible; it desugars to a normal application on previous versions

This requires changing the Hint Mode of the [Frame] type class because it should not fail if its parameter is an evar, but instantiate it instead. In order to prevent all the other instances of [Frame] to intantiate this evar themselves, we create a new type class [KnwonFrame], which corresponds to the old behavior.