Feature request: Add gf_gen_decode_matrix to the public API

ISA-L's public API provides no help to the user for generating a decode matrix for reconstructing missing data. Practically every single user of the erasure code module will need to do that. In fact, ISA-L's own test and benchmark programs all do this, copying similar code into 8 separate files. Could we please have a common helper function for it? As an example of such a function, see Jerasure's [jerasure_make_decoding_matrix](https://github.com/ceph/jerasure/blob/master/include/jerasure.h#L201).