store macro definitions in the function slot

store macro definitions in the function slot

Guile Emacs Lisp previously kept macros in a separate macro slot; now
macros are stored as macro objects in the function slot for
compatibility with other implementations.

* module/language/elisp/compile-tree-il.scm (macro-slot): Remove.
  (is-macro?): Check that the argument is a symbol. Now-unnecessary
  check removed in `compile-tree-il'.
  (macro?, define-macro!, get-macro): Store macro definitions in the
  function slot, not in a separate macro slot.
* module/language/elisp/runtime.scm (built-in-macro): Wrap the macro
  function in a macro object (i.e., cons the symbol `macro' onto it).
* module/language/elisp/runtime/function-slot.scm: Move contents to
  "subrs.scm". Re-export function and macro definitions instead of
  defining functions directly in this module.
* module/language/elisp/runtime/macro-slot.scm: Move contents to
  "macros.scm" and remove.
* module/language/elisp/runtime/macros.scm: New file containing macro
  definitions from "macro-slot.scm".
* module/language/elisp/runtime/subrs.scm: New file containing function
  definitions from "function-slot.scm".

Signed-off-by: Andy Wingo <wingo@pobox.com>