Marshalling:
* Fixes several silly bugs.
* Support the RawContents type.
* Support the RawValue type.
* Recurse into nested interface{}.

Both directions:
* Better handling of SETs. You can now tag an element in a
  structure with "set" to get the correct tag for serialisation.
* For types that aren't in a structure, you can now name them
  with "SET" on the end.
* SETs are no longer implicitly treated as SEQUENCEs.

R=rsc
CC=golang-dev
https://golang.org/cl/201049