Greatly simplify do_box_assign (any/enum class assign).
Greatly simplify do_box_assign (any/enum class assign).

The case of rhs being nil or containing a nil value has not been
possible since Lily started requiring a value for all assignments.

jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily,jesserayadkins/lily,FascinatedBox/lily