Pointer to the location of a sub-column within the stacked matrix.
This struct contains length information, but information from StackedLayout (namely l_skip)
is needed to determine if this is a strided slice or not.
Computes the Reed-Solomon codeword of each column vector of eval_matrix where the rate is
2^{-log_blowup}. The column vectors are treated as evaluations of a prismalinear extension on
a hyperprism.