Extract stable_unique to own function for clarity (VowpalWabbit#1559)

* Extract stable_unique function

* Fix unnecesarily complex type

* Move stable_unique to own header

* Add stable_unique test

* add project to vw.sln for unit tests

* Remove precompiled header from all configurations

unit_test/unit_test.vcxproj

@@ -203,4 +203,4 @@
     <Error Condition="!Exists('..\..\vowpalwabbit\packages\boost_program_options-vc140.1.63.0.0\build\native\boost_program_options-vc140.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\..\vowpalwabbit\packages\boost_program_options-vc140.1.63.0.0\build\native\boost_program_options-vc140.targets'))" />
     <Error Condition="!Exists('..\..\vowpalwabbit\packages\boost_unit_test_framework-vc140.1.63.0.0\build\native\boost_unit_test_framework-vc140.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\..\vowpalwabbit\packages\boost_unit_test_framework-vc140.1.63.0.0\build\native\boost_unit_test_framework-vc140.targets'))" />
   </Target>
-</Project>
+</Project>