implement setters support as described in #39

[PatrickLaflamme]

This PR implements setters support as described in #39.

Note: In run_setter_on_object, we do an expect on the optional setter_position field, as the FunctionType.call method requires a SpanWithSource. I looked into refactoring to change the setter_position from an Option but it looked like it'd have wide consequences. @kaleidawave - would love your thoughts on this one.