Refactor sprandn/sprand code, move common code to common function (bug #34352) * __sprand_impl__.m: New file * module.mk: Add new file * sprand.m: Remove comment in docstring about inaccuracy of density. Put sprandsym in @seealso. Refactor repeated code into __sprand_impl__.m * sprandn.m: Ditto. Also enable test for exact density.