Add a Boolean "not" operator.

Not wholly convinced this is a good idea, maybe "not" should be a
builtin function instead? 'not (.a == .b)' vs. '.a == .b | not'

c/compile.c

@@ -383,7 +383,11 @@ block gen_or(block a, block b) {
   block_append(&code, gen_condbranch(block_join(gen_op_simple(POP), gen_op_const(LOADK, jv_true())),
                                      if_a_false));
   return code;
-  
+}
+
+block gen_not(block a) {
+  return block_join(a, gen_condbranch(gen_op_const(LOADK, jv_false()),
+                                      gen_op_const(LOADK, jv_true())));
 }
 
 block gen_cond(block cond, block iftrue, block iffalse) {