Replace `OrdOffset` with `OffsetList`

[frankmcsherry]

The OrdOffset trait allowed us to slot in u32 or u64 in for usize for offsets, in order to save the RAMs. That isn't terrible, but we can remove the complexity, and the potential surprise if we go too large, with the OffsetList struct that generally behaves as a Vec<u32> until we reach a point at which the entries exceed its size, at which point it behaves like a not-great Vec<u64>.

[frankmcsherry]

Parting thought: if we ever don't like the opinionated lock-in of OffsetList, it should be easy to add back special cases that just use one type and panics when things don't work (to recover the prior implementation).