More constant folding: null, true, and false

A step towards doing constant folding for arrays and objects.

builtin.c

@@ -868,9 +868,6 @@ static block bind_bytecoded_builtins(block b) {
   {
     struct bytecoded_builtin builtin_defs[] = {
       {"empty", gen_op_simple(BACKTRACK)},
-      {"false", gen_const(jv_false())},
-      {"true", gen_const(jv_true())},
-      {"null", gen_const(jv_null())},
       {"not", gen_condbranch(gen_const(jv_false()),
                              gen_const(jv_true()))}
     };

parser.y

@@ -638,7 +638,15 @@ FORMAT {
   jv_free($2);
 } | 
 IDENT {
-  $$ = gen_location(@$, locations, gen_call(jv_string_value($1), gen_noop()));
+  const char *s = jv_string_value($1);
+  if (strcmp(s, "false") == 0)
+    $$ = gen_const(jv_false());
+  else if (strcmp(s, "true") == 0)
+    $$ = gen_const(jv_true());
+  else if (strcmp(s, "null") == 0)
+    $$ = gen_const(jv_null());
+  else
+    $$ = gen_location(@$, locations, gen_call(s, gen_noop()));
   jv_free($1);
 } |
 IDENT '(' Args ')' {