Promote function type aliases to the public module API.

Promote function type aliases to the public module API.

Previously module authors had to define type aliases for module
functions and finalizers themselves.  This commit adds and documents
aliases so that this is no longer necessary.

* src/emacs-module.h.in: Add 'emacs_function' and 'emacs_finalizer'
type aliases.

* src/emacs-module.c: Remove old 'emacs_subr' and 'emacs_finalizer'
type aliases.
(struct Lisp_Module_Function, module_make_function): Switch from
'emacs_subr' to 'emacs_function'.

* doc/lispref/internals.texi (Module Functions): Document and use
'emacs_function' type alias.
(Module Values): Document 'emacs_finalizer' type alias.

* etc/NEWS: Mention change.