Cleaner fe_frombytes() (loading field elements)

Cleaner fe_frombytes() (loading field elements)

The old version of fe_frombytes() from the ref10 implementation was not
as clean as I wanted it to be: instead of loading exactly the right
bytes, it played fast and loose, then used a carry operation to
compensate.

It works, but there's a more direct, simpler, and I suspect faster
approach: put the right bits in the right place to begin with.