Support watching of (collections of) reactive objects

[berendkleinhaneveld]

This allows for:

a = reactive({"foo": "foo"})

watcher = watch(a, cb, ...)

and

a = reactive({"foo": "foo"})
b = reactive(["bar"])

watcher = watch([a, b], cb, ...)

instead of:

watcher = watch(lambda: a, cb, ...)