Add support for custom formatted error messages in `*_that!`

[kupiakos]

Example usage:

for n in 0..100 {
    let something = unimplemented!();
    assert_that!(test_fn(n), eq(something), "for {n}"):
}

[hovinen]

We do have failure_message and with_failure_message which allow:

verify_that!(test_fn(n), eq(something)).with_failure_message(|| format!("for {n}")).unwrap()

Does that work for you, or do you want a more abbreviated version integrated into assert_that! et al.?

[kupiakos]

A more abbreviated version in args after the matcher would be strongly preferred - in part to make migration from assert_eq! and kin much more trivial.