collections.deque: Speedup `appendleft`, implement `__iter__` and `__str__`
unittest: Add `assertFalse` to `TestCase`
uasyncio.core: Doesn't depend on heapq module for some time now.
uasyncio.core: Follow builtin "time" module rename to "utime".
uasyncio.core: Release 0.8.3.
time: Add dummy "time" module, following renaming builtin to "utime".