Feature request: add ec_check

ISA-L's raid module includes the parity check functions `xor_check` and `pq_check` for verifying parity. But there's no equivalent in the erasure code module. Could we please have a general parity check function like `int ec_check(int len, int k, int rows, unsigned char *gftbls, unsigned char **data)`? Or perhaps a pair of functions, one that takes precalculates a parity check matrix, and one that uses it to verify parity?