An interrupt-based implementation of HardwareSerial

[michpro]

The currently available implementation of HardwareSerial is very primitive and is based on polling.
Especially when sending data, this causes unnecessary blocking of the program's execution of other tasks.
A DMA-based implementation, although it would relieve the CPU, will not be optimal for the variable amount of data transferred, hence I propose a traditional interrupt-based implementation.

[maxint-rd]

The current implementation is not just primitive polling, it's also also missing some Arduino core functionality such as available() and peek().

For my own projects I made up a polling routine to fill the buffer used to implement available() and peek().

@tanakamasayuki proposed PR #79 as a simple improvement to implement available() and also suggested using interrupts, illustrated with some code.

+1 for a nice interrupt based implementation. I'm in favor of anything better, but currently prefer small code size, since I found use of the limited flash on the CH32V003 the most prevalent issue in my projects...