[MLton] different word sizes

[Stephen Weeks]

One thing I was wondering as I was implementing this packing stuff was
if we were really gaining anything by having the codegen support Word8
and Word16 and the associated primitives.  That is, what if I were
just to represent those as Word32?  It would save various headaches in
the code that have to make sure and introduce the coercions between
sizes.  Plus, eliminating the coercsions would actually eliminate some
code and some runtime.  Is there a downside I'm missing?