Add support for manually defined DeepCopy methods

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).

pkg/api/unversioned/time.go

@@ -33,6 +33,13 @@ type Time struct {
 	time.Time `protobuf:"-"`
 }
 
+// DeepCopy returns a deep-copy of the Time value.  The underlying time.Time
+// type is effectively immutable in the time API, so it is safe to
+// copy-by-assign, despite the presence of (unexported) Pointer fields.
+func (t Time) DeepCopy() Time {
+	return t
+}
+
 // NewTime returns a wrapped instance of the provided time
 func NewTime(time time.Time) Time {
 	return Time{time}