isolation: Add the mechanics for stream isolation
At this commit, the IsolationInfo structure is introduced which contains IsolationFlag that dictate if a stream can be shared on a circuit with others.
In order to have a commit that builds, the IsolationMap is removed from crates/arti and thus empty isolation info objects are being used.
Next commits will add support for this in the arti SOCKS proxy effectively refactoring the IsolationToken single use to a more complex system of flags.
Part of #150 (closed)
Signed-off-by: David Goulet dgoulet@torproject.org