Add is (not) len ...

[luke-biel]

As an user of test case I want to be able to test for lenght of actual value within complex assertions, eg.:

#[test_case(b"1" => is len 1)]
fn test_this(s: &[u8]) -> &[u8] {
	s
}

This feature can use simple #expr.len() and assume user uses type that supplies len method