Revisit HAMT implementation

[cdata]

Early on, we forked https://github.com/filecoin-project/ref-fvm/tree/master/ipld/hamt and made an async version of it. This was very useful at the time, but two consequences that arose from this decision:

1. The API is not quite coherent with the rest of our IPLD-adjacent code
2. The HAMT construct itself is not Send + Sync, though it probably should be

We should revisit the implementation. We may want to write our own, or we may be able to use the one being built for rs-wnfs: https://github.com/wnfs-wg/rs-wnfs/tree/main/crates/fs/private/hamt