compile-assembly: use file-level constants table

compile-assembly: use file-level constants table

* module/language/glil/compile-assembly.scm (compile-assembly): Rework
  to handle toplevel-specific code generation here, instead of in
  glil->assembly.  Specifically, here we build a global constant table,
  and arrange for it to be the objtable of the toplevel thunk.

  (compile-program): New helper, compiles a <glil-program> and returns
  just the (load-program ...) form.

  (compile-objtable): New helper, generates assembly to build an object
  table, using some other constants table, and possibly recursing to
  `compile-program' for cached GLIL programs.

  (glil->assembly): Simplify, removing the toplevel? argument, and
  replacing the object alist with an objtable computed in a previous
  pass.  Adapt to the new form of the objtable, and to use
  compile-program and compile-objtable.