Remove or move non-CPython asyncio functionality

[dhalbert]

These things are in the MicroPython implementation of asyncio but not in CPython asyncio:

core.py:

• SingletonGenerator: maybe rename to _SingletonGenerator
• sleep_ms():omit, rename to _sleep_ms or move.
• IOQueue
• Loop -> _Loop

event.py:

• ThreadSafeFlag

funcs.py:

• wait_for_ms()

stream.py:

• Stream: serves for StreamReader and StreamWriter, maybe _Stream
• stream_awrite, Stream.aclose (?), Stream.awrite, Stream.awritestr: legacy uasyncio

task.py:

• most things could be underscored