HashMap (Objects/Dicts/etc) and HashSet

[dfellis]

Rather than implementing our own HashMap type on top of Array and the xxhash function, it's probably better to use the built-in mechanisms in Rust and Javascript -- not because I don't trust xxhash to be good and performant, but being able to share these things with the "native" platform will make interop easier.

Just basic set, get, has, len, and a toArray/iter mechanism (it may or may not make sense to have several of the Array functions target an Iterable interface that Array and HashMap both implement; not going to force this either way at the moment, since I don't want Iterable to become "re-inject infinite loops into Alan."