This fixes issue #187.

It now turns the goal into `P` and `<pers> Q`, which is dual to
`iDestruct`, which turns `P ∧ <pers> Q` into `P` and `□ Q`.

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

This fixes issue #182.

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

Fixes #156

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

Fixes #177

Fixes #176