Generalized Hash Trie

[jhellerstein]

A data structure used in optimal joins, as described in the Wang/Willsey/Suciu paper on Freejoin

[jhellerstein]

• [x] basic structure: Insert/Prefix Search/get()/iter()/recursive_iter()
• [x] lattice integration: Len, Extend, Merge traits
• [x] testing in state/lattice bimorphism

[rohitkulshreshtha]

https://github.com/hydro-project/hydroflow/pull/1308