Add liftEitherT to \/ instances.

Implemented as a new EitherTOps class, rather than adding as a function to \/, because doing it there needed to specify all three types (lifted monad, left, right) due to covariant types in \/ and invariant types in EitherT.

Test and compilation check added.
Not a big deal, but if we are just checking for compilation, we could probably omit the vals and just do
```scala
r.liftEitherT[Option]: EitherT[Option, String, Int]
l.liftEitherT[Option]: EitherT[Option, String, Int]
```