Revert "Altered invocation IL generation to use constrained virtual calls instead of boxing for value types, if possible."

This reverts commit a27aa012f39c0afc2ead48e57263f5a55cf5027b.
Merge pull request #127 from Unity-Technologies/verification

Revert IL generation change
Merge pull request #128 from Unity-Technologies/unity-staging

Revert IL generation change