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It now supports the specialization pattern `(H spat1 .. spatn)`, which first
recursively specializes the hypothesis `H` using the specialization patterns
`spat1 .. spatn`.

This needed minor rearrangement.

All the `env` operations are prefixed `env_`, so this is more consistent.

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

This follows the proof at https://en.wikipedia.org/wiki/L%C3%B6b's_theorem#Proof_of_L%C3%B6b's_theorem

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

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

Fixes #177