BUG: sparse: be more fussy about 64-bit index data types

- never downcast
- rename nnz to maxval in get_index_dtype
- CSR maxval=max(nnz, shape[1])
- CSC maxval=max(nnz, shape[0]),
- COO maxval=max(shape)
- DIA maxval=max(shape),
- BSR maxval=max(bnnz, bshape[1])