Allow sets to be dereferenced

[tsandall]

Today, queries cannot dereference sets (doing so results in a runtime error). It would be nice if this wasn't the case.

Instead of:

p[x], x.foo = "bar"

We could write:

p[_].foo = "bar"

To deal with this, we will bind set references to their value. We can explain this as s[x].foo = bar is syntactic sugar for s[x], x.foo = bar.

With respect to implementation, we must handle the following case:

p = {false}

p[x], ... # evaluation should not stop even though p[x] will be bound to false.