Implement oneof support.

This includes the code generation changes,
and the infrastructure to wire it up to the encode/decode machinery.

The overall API changes are these:
  - oneofs in a message are replaced by a single interface field
  - each field in a oneof gets a distinguished type that satisfies
    the corresponding interface
  - a type switch may be used to distinguish between oneof fields

Fixes #29.
diff --git a/proto/equal.go b/proto/equal.go
index d8673a3..5475c3d 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -154,6 +154,17 @@
 		return v1.Float() == v2.Float()
 	case reflect.Int32, reflect.Int64:
 		return v1.Int() == v2.Int()
+	case reflect.Interface:
+		// Probably a oneof field; compare the inner values.
+		n1, n2 := v1.IsNil(), v2.IsNil()
+		if n1 || n2 {
+			return n1 == n2
+		}
+		e1, e2 := v1.Elem(), v2.Elem()
+		if e1.Type() != e2.Type() {
+			return false
+		}
+		return equalAny(e1, e2)
 	case reflect.Map:
 		if v1.Len() != v2.Len() {
 			return false