add Envelope

[plastikfan]

when using a struct as the core type T with rx, T can not have pointer receivers. This is a bit of a burden for larger structs as more copying will be enforced. To mitigate this, the Envelope can be used to wrap the core T. This will enable the client to use pointer receivers on any custom struct defined for use with rx. It also has the benefit of satifying the ProxyField constraint on the client's behalf.