Make SegmentId signed i32

[gatesn]

When writing, we want to eagerly assign segment IDs, but some layouts want to ensure their data ends up near the footer. In these cases we allow a layout to push_back the segment. Since we don't know the total number of segments, we cannot assign an unsigned ID.

This breaking change (although no in practice because the existing u32s can be read as i32) allows the reader to resolve segement specs as negative positions from the end of the segment map.