dec: is from_raw_parts unsafe?

[benesch]

#61 got me thinking: if Decimal::<N>::from_raw_parts is provided with a digits value that exceeds the maximum number of digits that can be stored in lsu for the specified N, does memory unsafety result? We should investigate!

If it does, I think we have two options:

1. Mark from_raw_parts as unsafe and document the danger.
2. Add a check (or checks?) like those that were added in dec: add to/from raw parts using BCD-encoded versions of LSU #61 and see what the performance impact is.

cc @sploiselle