Currently, these implementations are hard-coded for certain array lengths.

This was necessary on old Rust versions in which

• const generics were not possible, and
• array traits were restricted to max lengths of 32.
  But since v1.47 this is no longer the case.

By enabling the consts feature on typenum, we get access to the U<const N: usize> type. And this is what lets us write a single implementation for From and for Into that will work for all lengths!