Make two different trait for ReedSolomon Encoder and Decoder

[drskalman]

The encoder and the decoder of RS code has enough different behavoirs which merit them different trait. That is for example includes the fact the decoder need to compute, store and reusing error polynomial which of no use for the encoder. Plus the decoder needs to verify various characteristics of the code to make sure it is valid before attempting to decode while the encoder should be able to encode no matter what raw data.