[5.5] Maintain original order of collection items with equal values when using sortBy()

[damiani]

(Revisits #21214, which was reverted in #21255)

This PR implements stable sort for sortBy(), as discussed in #21214, but approaches it another way that doesn't break when passed sort options like SORT_STRING, SORT_NUMBER', SORT_NATURAL`, etc.

The result: sortBy() will now preserve the original order of collection items which have identical values, and will work correctly with all sort options.

Fixes #21253

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Coincidentally, the instability of sortBy() also seems to be the cause of issues arising from the recent addition of route fallbacks, which ended up re-sorting routes in an unpredictable manner. So this PR should have the side-effect of fixing #21257 and #21258.

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[themsaid]

@damiani route matching on dev-master doesn't use sorting at all, so this PR can be merged separately in a later release.

[taylorotwell]

I got several errors and several test failures when running this locally on PHP 7.1. Here is one of them:

1. Illuminate\Tests\Routing\RoutingRouteTest::testOptionsResponsesAreGeneratedByDefault
   ErrorException: array_multisort(): Argument ReflectionException #1 is expected to be an array or a sort flag

/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Support/Collection.php:1358
/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Routing/RouteCollection.php:192
/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Routing/RouteCollection.php:164
/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Routing/Router.php:599
/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Routing/Router.php:578
/Users/taylor/Documents/Code/Laravel/framework/src/Illuminate/Routing/Router.php:564
/Users/taylor/Documents/Code/Laravel/framework/tests/Routing/RoutingRouteTest.php:371

[damiani]

Hmm, all tests, including testOptionsResponsesAreGeneratedByDefault and all RoutingRouteTests, are passing for me and passed in Travis. I'll try to reproduce, but I suspect initializing $values to ensure it's an array will solve it.