[RFC] Rearrange fields in struct vimvar (to fix issue #223)

As far as I can tell these changes should fix issue #223.

Since the vv_filler field is never accessed directly removing it should not have any negative impact.

I searched the code base for any issues that could arise from this change but since the memory layout should stay the same, nothing stood out to me. If I missed a critical case feel free to close this pull request.