(thread/new func &opt capacity flags)
Start a new thread that will start immediately. If capacity is
provided, that is how many messages can be stored in the thread's
mailbox before blocking senders. The capacity must be between 1 and
65535 inclusive, and defaults to 10. Can optionally provide flags
to the new thread - supported flags are:
* :h - Start a heavyweight thread. This loads the core environment
by default, so may use more memory initially. Messages may
compress better, though.
* :a - Allow sending over registered abstract types to the new
* :c - Send over cfunction information to the new thread.
Returns a handle to the new thread.