Changes

[REINDEX] Reference slots in type nodes now include edge to target type

Before this commit, reference slots in a type node only kept a string
with the instance type name of the type of the reference. This prevented
us from doing things like trying to precompute the type of the target
node from the type of the source node + the name of the reference.

Taking advantage of such an optimisation will require a reindex of
existing models, though.

In addition, a new SlotWrapper has been created, and the `x.type`
property for slots has been renamed to `x.instanceTypeName` which is
more accurate. `x.type` now returns a reference to the relevant type
node for reference slots, or `null` otherwise.