Remove or move non-CPython asyncio functionality

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

Start by comparing core.py, event.py, funcs.py, stream.py, and task.py with the corresponding CPython asyncio APIs. Review each checklist item and determine whether it should be removed, moved, or renamed, including the legacy stream methods and task.py symbols. Done means every listed item has an agreed disposition and the implementation consistently reflects it.

Written by the indexing model from the issue text.