Belay

Features
* Initial WebREPL support. See documentation and `examples/09_webrepl`. Performance is a bit lackluster, but should be acceptable for projects that don't need rapid response.
* Optimized some command/responses for less communication round-trips (slightly improves WebREPL performance).
* Allow `main.py` and `boot.py` in the sync folder. They don't actually impact Belay's ability to operate. Removed `SpecialFilenameError` that would have been previously raised if either of these were present.
* Added optional keyword argument `keep` to specify files on-device that should not be deleted if a corresponding file doesn't exist in the sync folder. Defaults to `["boot.py", "webrepl_cfg.py"]`, since these files may contain device-specific information.

Bug Fixes

* Fix sync hash-checking logic. Previously all files were always synced.
* Fix and improved erroneous message in `ValueError` raised by `Device.sync`

Internal
* Moved on-device code snippets into their own folder for easier maintenance.

Features
* Support for `bytes` and `set` (all python literals now supported) arguments and return values.

Features
* Support for multiple device `task` and `thread` decorators.
* Allow access of decorated functions from `task` and `thread`.

Misc Changes and Improvements
- Improve decorator stripping logic.
- Change belay prefix from `__belay_` to` _belay_`.
- Change `json_decorator` to `__belay_json` to reduce chance of name collision.
- Reduce whitespace in returned json data.
- Rename `hash_remote_file` to `__belay_hash_file` to reduce chance of name collision. Don't invoke via `task` to circumvent double prefix.

- Initial release of Belay.