Builtins: add Time namespace

[folex]

Time-related functions need their own namespace. Currently, they use Peer namespace. That's sub-optimal: Peer aspect should be about peering and networking. Op isn't suitable as well: it's for pure functions.

So I suggest to have the following Time namespace:

service Time("time"):
  timestamp_sec
  timestamp_ms
  sleep

timeout is renamed to sleep: it's more to the point. timeout is a misleading name.

And mark corresponding functions in Peer as deprecated.