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.h

@@ -29,6 +29,7 @@ block gen_assign(block expr);
 block gen_definedor(block a, block b);
 block gen_and(block a, block b);
 block gen_or(block a, block b);
+block gen_not(block a);
 
 block gen_cond(block cond, block iftrue, block iffalse);