Rewrite driver to use a state machine

This passes the responsibility for calling the Reactor between threads
instead of relying on main_thread to pick up the abandoned Reactor.
Threads that are waiting in block_on are likely to acquire the reactor
on their second wait, and the main thread is only woken if there are no
block_on threads polling.

This also adds support for disabling the async-io thread which allows
applications that only process I/O from within block_on to avoid the
needless thread creation.  It also supports designating the async-io
thread as the sole owner of the Reactor to avoid this pass-the-reactor
behavior, if that is undesired.

fixes smol-rs#40

Cargo.toml

@@ -19,7 +19,6 @@ concurrent-queue = "1.2.2"
 futures-lite = "1.11.0"
 log = "0.4.11"
 once_cell = "1.4.1"
-parking = "2.0.0"
 polling = "2.0.0"
 slab = "0.4.2"
 socket2 = { version = "0.4.0", features = ["all"] }