Expose Clock instance directly in order to get the current game time

[lordmauve]

Clock.t stores the elapsed time of the clock.

Due to the way we expose the clock in builtins, clock is a module. We could convert it to a Clock instance, in which case clock.t would be a way of accessing the time.

(We should explain in documentation that t is the commonly used mathematical variable for time, hence the short name.)

[Mambouna]

#337 would provide clock.time() as it's needed for pausing animations.