v0.2.0: Support for concurrent nodes and async tasks

This new release supports re-entrant nodes
(e.g. node A can call node B which in turns calls node A again)
and exposes some primitives that are common in Vibe.d program,
namely runTask and sleep to allow polling tasks.

Support for future is not here yet (runTask returns void)
but might come in the future.

Note that, since Vibe.d messes with the global scheduler instance
in std.concurrency, it's likely you will need to reset it to null
to avoid your tests hanging randomly.