[willow-ahrens]

fixes #992

adds a function xp.replace(array, old, new) which replaces value old with value new in array. This function is helpful in sparse kernels. for example, after loading a graph matrix, one may wish to replace the zero value with infinity if they wish to then implement a shortest-paths operation: https://github.com/SparseArrayProgrammingSuite/SparseArrayProgrammingSuite/blob/33b2949b4c0e960df8818e94406ed5bd970cc76c/src/saps/util/adjacency.py#L63.

The operation sparse.where(A != old, A, np.inf) needs to allocate a boolean mask to perform this operation, but a fused replace would be a good moment to signal to sparse arrays that they should change fill values explicitly, and could give an opportunity to do so in O(1) time if the explicitly stored entries are already known not to be fill.

This can also be a good way to fix exceptional values, like NaN -> 0 versus NaN -> Inf
https://github.com/SparseArrayProgrammingSuite/SparseArrayProgrammingSuite/blob/33b2949b4c0e960df8818e94406ed5bd970cc76c/src/saps/benchmarks/pcg.py#L1920