Change UnitTuple to work on 32&64-bit platforms

This will at least allow `SIUnits` to load and pass tests on a 32-bit platform.

**N.B.:  There are still overflow problems for all platforms with the very small and very large prefixes.**
Merge pull request #43 from rickhg12hs/more-32-bit-friendly

Change UnitTuple to work on 32&64-bit platforms