Fail-fast validation

[jprochazk]

To protect against primitive DoS attacks, it should be possible to halt further validation on the first error. Cheap checks (e.g. length, range) should be performed first, and expensive checks (pattern, custom) last. There should be a priority system so that custom checks can be put first if the user know that they are cheap.

priority wraps a a rule and changes its priority level:

#[derive(Validate)]
struct Test {
    #[garde(
        length(min=10, max=100),
        priority(
          custom(cheap_check),
          100
        ),
    )]
    value: String,
}

fn cheap_check(_: &str, value: &str, _: &()) -> Result<(), Error> {
  // ... something cheaper than the `length` check
  Ok(())
}

Rules should have a constant priority (e.g. range is cheap because it's O(1), contains is not because it is worst case O(N*M)). It should also be possible to designate some rules as having complexity that depends on their input, e.g. prefix priority will be based on the length of the value, and regex priority will be based on the complexity of the expression.