Generic big operators that are no longer tied to CMRAs.

Instead, I have introduced a type class `Monoid` that is used by the big operators:

    Class Monoid {M : ofeT} (o : M → M → M) := {
      monoid_unit : M;
      monoid_ne : NonExpansive2 o;
      monoid_assoc : Assoc (≡) o;
      monoid_comm : Comm (≡) o;
      monoid_left_id : LeftId (≡) monoid_unit o;
      monoid_right_id : RightId (≡) monoid_unit o;
    }.

Note that the operation is an argument because we want to have multiple monoids over
the same type (for example, on `uPred`s we have monoids for `∗`, `∧`, and `∨`). However,
we do bundle the unit because:

- If we would not, the unit would appear explicitly in an implicit argument of the
  big operators, which confuses rewrite. By bundling the unit in the `Monoid` class
  it is hidden, and hence rewrite won't even see it.
- The unit is unique.

We could in principle have big ops over setoids instead of OFEs. However, since we do
not have a canonical structure for bundled setoids, I did not go that way.