dillon1000/react

Fix Array#at and similar cases to capture if receiver is mutable

The previous PR helped me realize we weren't handling Array#at correctly. If the 
receiver is a mutable value its effect should be Capture and the lvalue effect 
needs to be Store. This PR updates the definition for Array#at to make the 
receiver Capture, and then updates inference to automatically set the lvalue 
effect to Store if _any_ argument (or the receiver) was Capture.