Initial hamts commit
Implement hamt-contains?
Implement two argument version of hamt-ref
Add first bunch of hamts tests
Add alist->hamt tests
Implement hamt->alist and hamt-fold
Add hamt->alist test
Add folding tests
Add hamt-delete tests
Fix insertion into collision nodes

- use equal? for comparing hashes
- chash is the hash, not collision-hash
- return a collision node, not an alist
Add collision test
Add hamt collision "stress test"
Recursive calls to handle-subtrie in insert should shift the hash
handle-leaf in insert should shift the leaf hash
Add missing iota function to utils
Rename remove to delete in (pfds hamts) to satisfy racket
Fix embarrassing mixup in hamt-contains? (added tests for good measure)
Add hamt-map
Update README to mention HAMTs
Add hamt-size and tests