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
Did you consider instead `unsafe.Sizeof(0) == 4`? I wonder if it's better because it's measuring the size of int instead of the architecture. I have no idea. I can't think of why it would matter one way or the other, though.