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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

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

See the discussion in #163.

It now no longer requires the modality to be absorbing by default; it
only should be absorbing when non-affine hypotheses have been cleared.

As reported by @jjourdan: framing now no longer back tracks on whether
to strip laters or not. When framing below a later, we now only make it
strip laters of the head of the frame.

This regression was caused by a bug in handling spec patterns.

We now use the `Maybe` prefix as also used for `Frame`: it indicates
whether progress has been made by stripping of a later or not.

In the same way that `iFrame` would succeed on said goals.

This made the definition much simpler, and provides support for all
specialization patterns (although most are not very useful in
combination with `iAssert`).