We need to use conversion whenever there are multiple return types. If the expression does not accept our supertype, then we can attempt to convert it, which will already handle safety checks for multiple return types

This fixes SimpleExpression not converting  possible return types that are not contained in the desired types array. For example, if an expression can return a Number or a String, and we want an Expression that is a Number or an World, it will now include converters for String->Number and String->World