In some ring-based models, we know that if the ring is not stale, then
all our bounds should fit within 32-bits. This change makes these
explicitly u32 rather than implicitly narrowing later on.
Bring hs_reset_stream(), hs_reset_and_copy_stream()'s functionality into
line with hs_close_stream() by accepting a NULL scratch if and only if
the match callback is also NULL, indicating that no matches should be
delivered.
