UART driver refactor

[fauxpark]

Description

Moves the existing uart.[ch] driver to drivers/, and adds a ChibiOS counterpart (which may need some QA from someone more versed in the ARM side of things).

It would also be good to get some puts() and gets() implementations added, if possible. And I'm not really sure what to do with uart_available() on ARM.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[fauxpark]

Working on AVR (tested Pro Micro), Bluepill F103, and Proton-C, though for the latter, USART2 is actually enabled by default, despite the fact that A9 and A10 (labelled TX and RX on the pinout)'s AFs are RX/TX for USART1. Should I change this?