Allow sets to be treated like objects/arrays

In the past, topdown would bind set[x] to true. This resulted in
confusion when people attempted to write joins with set elements.

With this change, topdown treats sets like objects/arrays, except that
set[x] is bound to x. This allows users to write queries that
dereference sets just like objects and arrays.

Also, fix handling of self-joins where previously a recursive binding
could be added. The "self-join" test case was added to cover this.

Fixes #243