Improve module function terminology

Improve module function terminology

Module functions were previously called "function environments" when
the functions created by module_make_functions were lambdas.  Now we
can adapt the terminology and rename "function environments" to
"module functions" everywhere.  This also removes the name clash
between "function environments" and "module environments."

* src/emacs-module.c (module_make_function): Adapt comment to reality;
stop using "function environment" terminology.
(funcall_module): Stop using "function environment" terminology.