Stabilize sort order for Remarkup block rules

Summary:
Fixes T10929. Currently, the ordering of Remarkup block rules is ambiguous (multiple rules may have the same priority number) and depends on sort stability (if elements with the same value have their ordered retained by the sort).

Sort stability changed between PHP5 and PHP7, so the result of functions like asort() changed too if some of the values are the same.

Currently, some tests fail and some edge-case beahviors differ under PHP7 because of this. Particularly, the Remarkup literal test case fails under PHP 7.1 locally, because <space><space>%%% gets interpreted as a code block instead of a literal block.

To fix this:

• Use msortv() with PhutilSortVector, which is a stable sort.
• Sort on <priority, ClassName>. The class name is guaranteed unique so this ordering is unambiguous.

Test Plan:
Previously, a literal block test case failed on PHP7.1 locally:

This test now passes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10929

Differential Revision: https://secure.phabricator.com/D17771