Sets and executes timeouts set during a tick.

All timeouts and intervals set during a tick were being scheduled to run
at delay + end-of-tick, instead of delay + time-of-outer-timeout.

Scheduled run-at times were shifted because currentTime was being
incremented before executing scheduled functions.

Additionally, the execute loop was iterating over a functions-to-run
array, created from scheduledFunctions before starting. Any changes to
scheduledFunctions were being ignored during the tick, and the next tick
would ignore any functions which should have been executed in the past.

The commit is a rewrite of DelayedFunctionScheduler, preserving the
public interface. Execution of scheduled functions updates currentTime
on each iteration, and each time takes the functions with the lowest
runAtMillis from the schedule, if they aren't higher than endTime.