Text display implementation

[alexandruradovici]

Implement the text display API.

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
• Temperature API - https://github.com/tock/libtock-rs/tree/master/apis/temperature
• Console API - https://github.com/tock/libtock-rs/tree/master/apis/console
• Buttons API - https://github.com/tock/libtock-rs/tree/master/apis/buttons
• microbit:v2 - https://microbit.org/new-microbit/