Add variance specifiers to a few interfaces.
Checkpoint:
  Building infrastructure for supporting variant casts of interfaces
Checkpoint:
  Unoptimized implementation of generic interface (in/out) variance for As/Is/Cast
Variant invocations work at a basic level and have some simple optimizations applied so they aren't completely slow.
Added two failing tests to the project.
Neat, MultipleGenericInterfaces2 passes now.