Rework defmacro

Now in terms of the more correct `fun`.
Use a SimpleNamespace for hissp.macros._macro_
Rename once-deftype to deftypeonce
Rename once-define to defonce again

docs/lissp_whirlwind_tour.rst

@@ -1828,18 +1828,20 @@ Lissp Whirlwind Tour
 
    (dir _macro_)
 
-   ;;; This is a copy of of the following module.
+   ;;; This is a copy of of the following namespace.
 
-   #> hissp.._macro_
-   >>> __import__('hissp')._macro_
-   <module 'hissp.macros._macro_'>
+   hissp.macros.._macro_
 
-   (dir hissp.._macro_)
+   (dir hissp.macros.._macro_)
 
    ;;; Notice its containing module. Take a minute to read its docstring.
 
    (help hissp.macros.)
 
+   ;;; As a convenience, hissp.__init__ imports it as well:
+
+   hissp.._macro_
+
    ;;; The macros will still be available from there even if you clobber
    ;;; your _macro_ copy. Recall that you can invoke macros using their
    ;;; fully-qualified names.