and show that this is an instance of evaluation contexts

Just because we don't have such comments for other files, doesn't mean we can't
start applying some best practices.

This required a new ectx axiom: Positivity of evaluation contexts. This axiom was
also present in the old Iris 1.1 development, back when it still derived lifting
axioms for ectx languages.

Also remove some superfluous map_ prefixes.

Also, slightly reorganize.

Improve names, simplify definition of dist.

Shorten proofs and name variables that we introduce.

Requires should always be at the top of a file, Imports can be inside
of submodules.

It is strange that a Require in a submodule is allowed and then triggers
an error when using the whole module. This may be a Coq bug.