Allow type-checking type variables

I often see people trying to check the type of a type variable by doing `if (T is int) ...`.
This doesn't work, the expression `T` evaluates to a `Type` object which is not of type `int`.
The idea has merit, though.

We could perhaps special case `typeVariable is type` to actually check whether the type of `typeVariable` is a subtype of `type`. The expression is useless otherwise, so it's not like we will break any reasonable program, and users obviously find the syntax intuitive, since they keep using it.