Switch ThreadPoolScheduler to use nqp::timer op.
Pass an arg for the async handle to nqp::timer.

It's already required on Moar in order to use nqp::timer at all. Will
need some JVM work to get this in place also.
Ensure there's a thread for timer events.

Also, a minor simplification.
Fix AST-gen for blocks in hash constructors.

Need to migrate inner blocks out to the right scope, given that the
hash constructor does not imply one. Fixes "on" with the EnumMap
case on MoarVM, and no doubt some other things.
Merge branch 'async' into nom
Bump to an NQP with portable timer support.