Define traits for table access and iteration

[molpopgen]

No description provided.

[molpopgen]

I'm going to close this because:

1. No one is asking for it.
2. The dyn compatibility is tricky
3. The current PR is probably over-engineered and it would be far better, perhaps, to have traits that define an API for "return reference to tables" and "return reference to a tree sequence" that may be useful for clients encapsulating these types. But here, too, no one has been asking about this, so it seems like a YAGNI idea for the time being.