[formulas] Use in for scoping, not has() checks

[LeaVerou]

Currently, the in operator is mapped to the has() function. However, that's a bit of a waste for such a nice short operator.

I think it would be much better used for scope() which (once implemented properly) is a very powerful concept and worth exposing with an operator. Currently marking as blocked, since we're not there yet, but we should probably deprecate the in operator already, to prepare.

One downside is that the order of operands would be the reverse of what it is with the dot operator, e.g. it's student.name but name in student. But I think that's okay, since it's weaker than the dot operator (or at least it should be — again, bugs).