Use unboxed representations of Word32 and Int32 values

[melsman]

Currently Word32 and Int32 values are represented boxed. Some benchmarks (e.g., DLX) may benefit from using an unboxed representation of Word32 values.

Also, conversions like Word.fromLargeWord o Word32.toLargeWord are problematic as they involve boxing! Perhaps we can find a better way to deal with these conversions...