Support quitting in modules

Support quitting in modules

The idea is that modules should call env->should_quit from time to
time and return as quickly as possible if it returns true.

* src/emacs-module.c (module_should_quit): New module function.
(initialize_environment): Use it.
(funcall_module): Process potential pending quit.

* src/eval.c (maybe_quit): Add reference to module_should_quit.