In bringing back Clayton's PR piece-by-piece this was almost as easy to
implement as his version, and is much more like what I think we should be
doing.

Specifically, any time which defines a .DeepCopy() method will have that method
called preferentially.  Otherwise we generate our own functions for
deep-copying.  This affected exactly one type - resource.Quantity.  In applying
this heuristic, several places in the generated code were simplified.

To achieve this I had to convert types.Type.Methods from a slice to a map,
which seems correct anyway (to do by-name lookups).