run FEC decoding test in a loop

changelog

• refactor
  k
  and
  n
  into variables
• run the decoding FEC test in a loop from
  (k - 1) \rho + 1
  to
  k \rho
  , where
  \rho
  is the curve number of bytes per scalar field element, e.g.
  31
  for BLS-12-381
• add a NOTE about having too few bytes and elements, i.e. below
  (k - 1) \rho + 1
• add a FIXME about having too many bytes and elements, i.e. above
  k \rho