For this patch series to be bisectable, the previous patches left behind
some features, or temporarily added features, that we couldn't remove
until they are no longer use. Now we can finally delete these unused
features:

1. After the previous patches stopped using clock::get()->time() in sched.cc,
we can drop the alias osv::clock::get() we added temporarily just to make
it easier to compile the first patches in this series.
2. Drop the clock_event::set(s64) variant.
3. Drop the timer_base::set(s64) variant.

Fixes #81.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>