Add ResultExt with assume_ok helper

[bschoenmaeckers]

I found myself using prop_assume!(result.is_ok()) a lot before unwrapping some Result value that I use while testing. So to make life easier I come to the following api addition.

This allows to write test like the following.

use jiff::civil::Date;
use proptest::prelude::*;

proptest! {
  #[test]
  fn test_some_date(
    year in 1i16..=9999i16,
    month in 1i8..=12i8,
    day in 1i8..=31i8
  ) {
    // Reject invalid dates
    let date = Date::new(year, month, day).assume_ok()?;
    // ...rest of test...
  }
}

[rexmas]

I think this is a nice ergonomic addition, but ResultExt is pretty common. can we name this ProptestResultExt instead?

also seems like a good case for a sealed trait
e.g. from blog post

mod private {
    pub trait Sealed {}
}

pub trait SealedTrait : private::Sealed {
    fn method(&self);
}

[rexmas]

what do you think about naming prop_assume_ok?