Reflect Struct QOL

The bevy_reflect::Struct trait has taken ownership of the previously bevy_reflect::DynamicStruct::index_of and is now bevy_reflect::Struct::index_of_name. Most utility methods existed already on Struct, except for the method to get a fields index by name.

The bevy_reflect::FieldIter iterator had its items changed, from &dyn PartialReflect to a tuple of (&str, &dyn PartialReflect), so that you can iterate the names alongside the fields.