Encoding needs to be aware of lazy-loading

[aboodman]

Even after #11 is implemented, we still have to do more work in the encoding path, otherwise we end up re-expanding everything when trying to save. For example:

v, _ := ReadValue(<bigval>, cs)
v = v.Append(Int32(42))
// At this point,we've only read one ref

// This line ends up reading all the refs because it walks the tree in order to serialize :-/.
r, _ := WriteValue(v, cs)

[aboodman]

lgtm!