This is needed to fix RustBelt-related. Since type classes are no longer
over-eagerly resolved, the `biIndex` now sometimes contains an evar.

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

If the accessor introduces a binder, the first Coq-level intro pattern of `iInv`
is used for that binder unless the type of the binder is unit, in which case
`iInv` removes it completely.  Binders on the closing view shift are not (yet)
supported as they are harder to smoothly eliminate in the unit case.

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

rename : affinely_persistently -> intuitionistically. Add lemma about monpred_at and intuitionistically.

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

Split [FromAssumption] into three, depending on wheter the parameters are evars. This is to avoid loops in TC search.

sed -i 's/∀ᵢ/\<obj\>/g; s/∃ᵢ/\<subj\>/g' $(find ./ -name \*.v)

sed -i 's/absolute/objective/g; s/relative/subjective/g; s/Absolute/Objective/g; s/Relative/Subjective/g' $(find ./ -name \*.v)

Based on an earlier MR by @jung.

This change is slightly more invasive than expected: in monPred we were
using the embedding before the BI was defined. With the new setup, this
is no longer possible, because in order to make an instance of the
embedding, we need to know that `monPred` is a BI. As such, we define
`emp`, `⌜ _ ⌝` and friends directly in the model of `monPred` and later
prove that they are equal to a version in terms of the embedding.

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.

See the discussion in #163.