Simplify Clock/ClockMark design

- Do not require initialElapsed parameter
- Introduce instead ClockMark +- Duration operators
- Do not require ClockMark to expose its originating Clock back
- Turn elapsedFrom property into elapsed() function
- Rename abstract clock classes
- Fix unit of abstract clocks when constructing them

Add top-level docs for Clocks

libraries/stdlib/jvm/src/kotlin/time/MonoClock.kt

@@ -5,8 +5,7 @@
 
 package kotlin.time
 
-public actual object MonoClock : LongReadingClock(), Clock { // TODO: interface should not be required here
-    override fun reading(): Long = System.nanoTime()
+public actual object MonoClock : AbstractLongClock(unit = DurationUnit.NANOSECONDS), Clock { // TODO: interface should not be required here
+    override fun read(): Long = System.nanoTime()
     override fun toString(): String = "Clock(System.nanoTime())"
-    override val unit: DurationUnit = DurationUnit.NANOSECONDS
 }