REv3 idea: Support for interleaved stdout/stderr

Both in #173 and #188 we've attempted to add support for interleaved stdout and stderr to REv2. Even though we haven't been able to drag this across the finish line, it does show that there is interest in having a feature like that added to the protocol. Let's make sure we reconsider this as part of REv3.

Start by reviewing the attempted work in issues #173 and #188, then examine how REv3 should address interleaved stdout and stderr. Done means producing an agreed protocol design that explicitly supports this feature in REv3.

Written by the indexing model from the issue text.