kernel/util: Add heap implementation
kernel: Add another penalty for CPU bound threads

Each thread has its minimal priority that depends on the static priority.
However, it is still able to starve threads with even lower priority
(e.g. CPU bound threads with lower static priority). To prevent this
another penalty is introduced. When the minimal priority is reached
penalty (count mod minimal_priority) is added, where count is the number
of time slices since the thread reached its minimal priority. This prevents
starvation of lower priorirt threads (since all CPU bound threads may have
their priority temporaily reduced to 1) but preserves relation between
static priorities - when there are two CPU bound threads the one with
higher static priority would get more CPU time.
kernel: Remove possibility to yield to all threads

Kernel support for yielding to all (including lower priority) threads
has been removed. POSIX sched_yield() remains unchanged.

If a thread really needs to yield to everyone it can reduce its priority
to the lowest possible and then yield (it will then need to manually
return to its prvious priority upon continuing).
libroot: Update sched_get_priority_{max, min}()

SCHED_RR is a real-time scheduling policy.
SCHED_FIFO and SCHED_SPORADIC are not supported (at least for now).