This fixes issue #182.

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

We do this in two ways:

- Use `notypeclasses refine` instead of `eapply`, to avoid type class
  search being called arbitrary.
- Use `typeclasses eauto` instead of `apply _`, to avoid type class
  search being called on unrelated evars.

I mainly tried this for `iSpecialize` and friends; this same remains to
be done for all other tactics.

This commit also makes partial progress w.r.t. issue #135.

Based on an earlier MR by @jung.

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 requested by @jtassaro.

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.