mutator methods

[dvanhorn]

We've moved away from the (field f) form of referencing fields and instead have used automatically generated accessor methods: (send this f).

We should do the same for (set-field! f x). I propose we generate mutator methods (send this f! x).

There is some potential for this going wrong, eg if you have a class with fields f and f!, then method f! is ambiguous, but so long as this is a syntax error, it seems fine.