Reference implementation

[zenhack]

We should have a reference implementation of the spec, whose implementation is as close to the declarative specification as possible; this will be useful when validating other implementations, and testing that the spec does what we think it does.

I moved the spec repo to an organization account, so we can have related projects like this in their own repos while still keeping them organized.

[zenhack]

I did a first pass on this; there's a new repo called 'reference-implementation'. Needs testing, doesn't implement the tree algorithm yet, but I'm going to close this and open relevant issues there.