1) Fixed a significant bug in MutablePriorityQueue when setting

the indices allowing rescheduling of values in the middle of the queue.
2) Implemented a cache friendly variant - MutableSkipHeapPriorityQueue
   based on
   https://playfulprogramming.blogspot.com/2015/08/cache-optimizing-priority-queue.html
   https://github.com/rollbear/prio_queue/blob/master/prio_queue.hpp

tests/libslic3r/CMakeLists.txt

@@ -12,6 +12,7 @@ add_executable(${_TEST_NAME}_tests
 	test_placeholder_parser.cpp
 	test_polygon.cpp
 	test_mutable_polygon.cpp
+	test_mutable_priority_queue.cpp
 	test_stl.cpp
 	test_meshsimplify.cpp
 	test_meshboolean.cpp