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/testdata

@@ -260,3 +260,7 @@ def inc(x): x |= .+1; inc(.[].a)
 [false,true]
 [false,true]
 [false,false]
+
+[.[] | not .]
+[1,0,false,null,true,"hello"]
+[false,false,true,true,false,false]