BUG: sparse: fix some integer wraparounds in sparsetools

Some of the sparsetools routines index into 2D dense arrays.  The
indices into these are not bounded by nnz, and can overflow int32
limits. Take care of this.
BUG: sparse: fix int32 wraparounds in BSR matrices

The issue here is that the nnz of BSR is not bounded by the index data
type, which only bounds the number of blocks.  The block size may easily
overflow int32.
BUG: sparse: fix integer overflow in CSR matmat
TST: sparse: add tests probing for int32 overflows
DOC: document how to run slow tests
Merge pull request #3224 from pv/sparsetools-overflow

BUG: sparse: fix int32 overflows in sparsetools