adds a map (but lets call it a pool) of all
running timers. this makes us able to keep
track of all running intervals and timeouts.
it also means hilbish can wait for them to
be done before exiting (it only waits when
non interactive).
this introduces the `hilbish.timers` interface,
documented by `doc timers`. the `hilbish.interval`
and `hilbish.timeout` functions return a timer
object now.
TorchedSammy
