Option to add "comparison" to `AssertionFailed`

[kbond]

When the PHPUnit handler is being used, use PHPUnit's comparison api to generate the standard diff:

Code to create the proper diff:

try {
    SebastianBergmann\Comparator::getInstance()
        ->getComparatorFor($expected, $actual)
        ->assertEquals($expected, $actual)
    ;
} catch (SebastianBergmann\Comparator\ComparisonFailure $e) {
    $diff = $e->getDiff();
}

Requirements for $expected/$actual to be "diffed":

• both arrays
• both strings
• both objects of the same class

Alternative: some kind of phpunit-only hook?