Alternate to input and usys.stdin on Move hub

[dlech]

Is your feature request related to a problem? Please describe.
This has come up several times recently (#1099, #1085, #529). There is no easy way to get input from a computer to a BOOST Move hub.

Describe the solution you'd like
A function similar to usys.stdin.read() that plays nice with the new async run loop with minimal implementation size.

Describe alternatives you've considered

• The new broadcast/observe functionality is not a complete solution since it can't be used on macOS and requires experimental BlueZ features to be manually enabled on Linux (or elevated privileges to directly poke HCI).
• We have also proposed an API for wrapping the Nordic UART service, but the Move hub currently doesn't have anywhere near enough free flash memory to implement this.
• We could implement a very minimal mp_hal_readline() (e.g. backspace wouldn't work) and use the existing builtin input() function. However, since this prints in addition to getting input, it would not be ideal for use in a program with an async run loop where we need it to be awaitable rather than blocking.

[laurensvalk]

We've previously had a non-blocking getchar that seemed to work well for most people.

See #154

[dlech]

👍 That is probably as simple as it gets.

[dlech]

The only downside I see is that to be non-blocking it could not support multi-byte UTF-8 characters (and would probably have to return bytes or int instead of str to avoid exception in case we ever enable MICROPY_PY_BUILTINS_STR_UNICODE). That is, unless we implemented a way to peek at the stdin buffer without removing bytes.