HardwareSerial: re-implement based on ESP-IDF UART driver

[igrr]

To make porting the core to the new chips easier, HardwareSerial should be implemented on top of the ESP-IDF driver. Task is to:

• Investigate if:

  1. ESP-IDF uart driver provides sufficient functionality for HardwareSerial to work
  2. doesn't require significantly more resources than the current implementation
  3. doesn't result in significantly worse performance in common scenarios

  If the answer to any of these is no, consider using uart_hal from ESP-IDF hal component, instead.

• Port HardwareSerial to the UART driver (or hal) and add test coverage.