- Every message has a new field containing a list of unknown fields:
  ```unknownFields :: Message msg => Lens' msg [TaggedValue]```
- Unknown fields are preserved by `decodeMessage`, `encodeMessage`,
  and `showMessage`
- Unknown fields still cause an error for `readMessage`.

A few TODOs:
- For now, unknown groups are printed sub-optimally by `showMessage`: the
  start/end group tags (and everything in between) all get displayed as
  individual fields, rather than being organized into a sub-struct.
- The `discardUnknownFields` function isn't recursive, unlike in other
  languages.
- The Ord instance doesn't try to do anything special, just treating
  the unknown fields as a list of values.  If it really matters then
  `discardUnknownFields` can help resolve the ambiguity.

It's needed by the benchmarks.