Revert "Remove the domain finiteness hypothesis for the function CMRA, and put cmra_extend in Type."

This reverts commit fa897ff5.

The finiteness was needed to have the axiom of choice over the domain. This axiom is not needed if cmra_extend is in Type.

Stronger `iNext` that performs arithmetic cancelation

Closes #148

See merge request FP/iris-coq!109

Fixed by stdpp 93b4ec70e13a573a9055a5bf1269f5885e18e843.

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`).

Thanks to @jtassaro for reporting this problem.