Add the Buzzer API and music example

[alexandruradovici]

Based on the LED API present here, implement the Buzzer API and port the music example from libtock-c. You can use a micro:bit v2 as hardware.

Documentation:

• libtock-rs overview - https://github.com/tock/libtock-rs/blob/master/doc/Overview.md
• Tock system calls - https://github.com/tock/tock/blob/master/doc/Syscalls.md
• LED API - https://github.com/tock/libtock-rs/tree/master/apis/leds
• The music example - https://github.com/tock/libtock-c/tree/master/examples/music
• microbit:v2 - https://microbit.org/new-microbit/