The compile-time calculation for `factor` overflows because `1<<31`
cannot be represented by `int`.  By casting to `uintmax_t` first, we
can avoid this overflow.

Closes #380

This assertion currently fails for S32 due to integer overflow (#380).

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Refactor the conversion functions to classes and pass an instance to
the new function AllocateConvert().