Add lyng.observable ObservableList hooks, flow events, and docs

docs/ObservableList.md

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+# ObservableList module
+
+`ObservableList` lives in explicit module `lyng.observable`.
+
+Import it first:
+
+    import lyng.observable
+    >>> void
+
+Create from a regular mutable list:
+
+    import lyng.observable
+    val xs = [1,2,3].observable()
+    assert(xs is ObservableList<Int>)
+    assertEquals([1,2,3], xs)
+    >>> void
+
+## Hook flow
+
+Event order is:
+1. `beforeChange(change)` listeners
+2. mutation commit
+3. `onChange(change)` listeners
+4. `changes()` flow emission
+
+Rejection is done by throwing in `beforeChange`.
+
+    import lyng.observable
+    val xs = [1,2].observable()
+    xs.beforeChange {
+        throw ChangeRejectionException("no mutation")
+    }
+    assertThrows(ChangeRejectionException) { xs += 3 }
+    assertEquals([1,2], xs)
+    >>> void
+
+## Subscriptions
+
+`beforeChange` and `onChange` return `Subscription`.
+Call `cancel()` to unsubscribe.
+
+    import lyng.observable
+    val xs = [1].observable()
+    var hits = 0
+    val sub = xs.onChange { hits++ }
+    xs += 2
+    sub.cancel()
+    xs += 3
+    assertEquals(1, hits)
+    >>> void
+
+## Change events
+
+`changes()` returns `Flow<ListChange<T>>` with concrete event classes:
+- `ListInsert`
+- `ListSet`
+- `ListRemove`
+- `ListClear`
+- `ListReorder`
+
+    import lyng.observable
+    val xs = [10,20].observable()
+    val it = xs.changes().iterator()
+    xs[1] = 200
+    val ev = it.next()
+    assert(ev is ListSet<Int>)
+    assertEquals(20, (ev as ListSet<Int>).oldValue)
+    assertEquals(200, ev.newValue)
+    it.cancelIteration()
+    >>> void