Introduce if! & when! macros

The condition argument is evaluated at compile/expansion time, and its
body expanded directly.

core/core-lib.el

@@ -240,6 +240,23 @@ writes to `standard-output'."
                 (save-silently t))
             (prog1 ,@forms (message ""))))))
 
+(defmacro if! (cond then &rest body)
+  "Expands to THEN if COND is non-nil, to BODY otherwise.
+COND is checked at compile/expansion time, allowing BODY to be omitted
+entirely when the elisp is byte-compiled. Use this for forms that contain
+expensive macros that could safely be removed at compile time."
+  (declare (indent 2))
+  (if (eval cond)
+      then
+    (macroexp-progn body)))
+
+(defmacro when! (cond &rest body)
+  "Expands to BODY if CONDITION is non-nil at compile/expansion time.
+See `if!' for details on this macro's purpose."
+  (declare (indent 1))
+  (when (eval cond)
+    (macroexp-progn body)))
+
 
 ;;; Mutation
 (defmacro appendq! (sym &rest lists)