AK: Introduce MonotonicTime

This class takes on the duties of CLOCK_MONOTONIC, a time without a
defined reference point that always increases. This informs some
important design decisions about the class API: MonotonicTime cannot be
constructed from external time data, except as a computation based on
other monotonic time, or the current monotonic time. Importantly, there
is no default constructor, since the reference point of monotonic time
is unspecified and therefore without meaning as a default.

The current use of monotonic time (via Duration) includes some potential
problems that may be caught when we move most to all code to
MonotonicTime in the next commit.

The API restrictions have one important relaxation:
Kernel::TimeManagement is allowed to exchange raw time data within
MonotonicTime freely. This is required for the clock-agnostic time
accessors for timeouts and syscalls, as well as creating monotonic time
data from hardware in the first place.

AK/Time.cpp

@@ -227,6 +227,16 @@ Duration Duration::now_monotonic_coarse()
     return now_time_from_clock(CLOCK_MONOTONIC_COARSE);
 }
 
+MonotonicTime MonotonicTime::now()
+{
+    return MonotonicTime { now_time_from_clock(CLOCK_MONOTONIC) };
+}
+
+MonotonicTime MonotonicTime::now_coarse()
+{
+    return MonotonicTime { now_time_from_clock(CLOCK_MONOTONIC_COARSE) };
+}
+
 UnixDateTime UnixDateTime::now()
 {
     return UnixDateTime { now_time_from_clock(CLOCK_REALTIME) };