Add a convenience type for properties involving equivalence

[neithernut]

This type was originally motivated by observing that a class of
properties will involve a pseudo-identity followed by a check for
equivalence between its input and output. A generalization of would be
properties which are defined by the equivalence check, only. See #280.

For this class, we usually want to know how those two values differ,
rather than only that they do. Test-authors may thus write tests like
the following in order to include those values in a failure report:

fn revrev(xs: Vec<usize>) -> TestResult {
    let rev: Vec<_> = xs.clone().into_iter().rev().collect();
    let revrev: Vec<_> = rev.into_iter().rev().collect();
    if xs == revrev {
         TestResult::passed()
     } else {
         TestResult::error(format!("Original: '{:?}', Identity: '{:?}'", xs, revrev))
     }
}

This change introduces a convenience type which encapsulates the
equivalence check as well as the error message generation. Using it, the
above test could be written as:

fn revrev(xs: Vec<usize>) -> Equivalence<Vec<usize>> {
    let rev: Vec<_> = xs.clone().into_iter().rev().collect();
    let revrev: Vec<_> = rev.into_iter().rev().collect();
    Equivalence::of(xs, revrev)
}