Make "not" a builtin function rather than syntax.

Fixes a really awkward grammar issue.

c/parser.y

@@ -53,7 +53,6 @@
 %token END "end"
 %token AND "and"
 %token OR "or"
-%token NOT "not"
 %token SETPIPE "|="
 %token SETPLUS "+="
 %token SETMINUS "-="
@@ -210,10 +209,6 @@ Exp "and" Exp {
   $$ = gen_and($1, $3);
 } |
 
-"not" Exp {
-  $$ = gen_not($2);
-} |
-
 Exp "//" Exp {
   $$ = gen_definedor($1, $3);
 } |