Support VARIANT on 32-bit and 64-bit architectures

* Changed definition of VARIANT struct so it matches
the 32-bit and 64-bit versions of the Windows VARIANT type
depending on GOARCH (i.e. use `int` instead of `int64` so that
the size is correct for both `386` and `amd64`)
* Added NewVariant constructor to simplify VARIANT creation
and handle special-cases when running on 32-bit related
to struct layout differences on 32-bit vs. 64-bit
@mgspross I couldn't answer that. I started my own fork in order to do some refactoring and that was merged in. I say the cleaner the code the better. Biggest problem is having it still work with everything. Backwards compatibility would have to be maintained at this point, I think.