The only drawback is that we have to restrict iprod to finite types,
but that is fine.

Thanks to Amin Timany for the suggestion.

So, only use the type class for contractive functors.

Add both non-expansive and contractive functors, and bundle them for the general Iris instance as well as the global functor construction

This allows us to move the \later in the user-defined functor to any place we want.
In particular, we can now have "\later (iProp -> iProp)" in the ghost CMRA.

Since functor instances are just used as combinators, there is really
no need for functors that are not contractive.

make the global functor stuff in the various constructions more uniform; change it such that barrier/proof does not have to repeat the functors it needs

add a version of [cancel] that works with goals of the form [_ |- pvs _]; and use that for the barrier proof

write tactics to move particular assertions to the front, and to introduce a (*) while taking paticular assertions to the left/right

For consistency's sake.

This cleans up some ad-hoc stuff and prepares for a generalization
of saved propositions.