Add time.Timer, time.Ticker and convenience functions time.tick,
time.after and time.after_func. These will be used in context to support
deadlines and timeouts.

While at time topic, also provide sleep and now from golang.time, so
that there is no need to import both golang.time and stdlib's time in a
file.

Provide time constants in the module as they are useful to have and
mirror constants provided by Go's time.

Note: timers implementation is very suboptimal for now.