Restore simple fe_frombytes()

Restore simple fe_frombytes()

Though I think the option of choosing how many bits we mask off at the
end is useful, it made the code more confusing. Better leave it for the
one case where we really need it: parsing Elligator representatives,
which are represented with one less bit than usual.

This does cost us 4 lines of code, but I think they're worth it. The
previous code was too much like code golf for my comfort.