This is in preparation for coq/coq#9274.

This lemma is similar to `later_ownM`.

Use explicit names in some scripts, re-organize fupd plainly derived laws, adjust wsat import/export.

Modify adequacy proof to not break the 'fancy update' abstraction. Modify fupd plainly interface and add new derived results.

The BI interface is then instantiated using these laws.  In particular, this
shows that the rules we claim to be admissible actually are.

Thanks to @dfrumin.

This version allows one to either close or cancel the invariant after opening it.

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.

The closing view shift's LHS mask is now universally quantified, which
makes it easier to execute the closing view shift.