Should we be using FromJSValConvertible for enum conversions?

[Manishearth]

It seems like we don't have FromJSValConvertible implementations for enums. I can't think of a good reason for doing this, and this also means our bindings code is littered with code like

        let arg0: XRReferenceSpaceType = match find_enum_value(*cx, HandleValue::from_raw(args.get(0)), XRReferenceSpaceTypeValues::pairs) {
            Err(_) => { return false;
         },
            Ok((None, search)) => { throw_type_error(*cx, &format!("'{}' is not a valid enum value for enumeration 'XRReferenceSpaceType'.", search)); return false;
         },
            Ok((Some(&value), _)) => value,
        };

We should probably fix this.

Furthermore, this prevents us from using sequence<EnumType> in dictionaries in WebIDL