supporting a pseudo-zero construct on cusp

[paldar]

cuspbackend::matrix constructor is slightly different from that of sequential::matrix (and frontend::matrix, for that matter) in terms of how zeroes are treated. need to fix this.

however, if the only time a structural zero is used is at negation, and negate needs to take a semiring to get its ones and zeroes, there is really no point in storing a separate zero at the matrix level, eh?

[paldar]

storing structural zero at construction. used as reference only.