Add is (not) empty

[luke-biel]

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

#[test_case("1" => is empty)] // fails
#[test_case("" => is empty)] // succeeds
fn test_this(s: &str) -> &str {
	s
}

We can rely on #expr.is_empty() and that user supplies a type that has such function.