Refactoring, formatting, and CI

[jessebraham]

This PR is a little all over the place, but essentially:

• Fixed error message in build script
• Consolidated the print! and println! macro implementations
• Moved each implementation of the Write trait into a module, along with any required constants (fixes UART_TX_ONE_CHAR is never used when using RTT #21 and just generally cleans up the #[cfg]s)
• Add a rustfmt.toml configuration file (closes Add an autoformatter #24)
• Add a CI workflow which builds the library in each possible configuration and checks the code formatting

Successful CI run can be seen here on my fork.

[bjoernQ]

Finally, CI checks! LGTM