Merge: lib/core/time: more time related services

Three things here:
* Introduce `Int::sleep` because of my hatred for `1.0.sleep`
* Introduce `get_microtime` because timestamp are better in microseconds
* Introduce `get_millitime` because sometimes, god forbid, we have to deal with Javascript

Also, I'm using `sys/time`, pretty sure I'm doing something that should not be in the core library.

Pull-Request: #2436