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Initial support for WinChipHead WCH-Link RISC-V

WCH's RISC-V MCUs, such as CH32V103, have a proprietary
2-wire debug interface. The vendor provided a custom
debug adapter called WCH-Link, with a closed-source
fork of OpenOCD(violates GPL) embedded in their IDE
software called MounRiver(`http://www.mounriver.com/download`).

The vendor's OpenOCD communicates with WCH-Link using a minimal
protocol, payloading RISC-V DM(Debug Module) commands directly,
shadowing the transport layer completely, which also makes
the port quite ugly.

This is an initial attempt to add support for WCH-Link
by reverse-engineering the vendor's OpenOCD.

Actual debugging isn't possible yet as I'm currently leaving
out some (weird?) initial command sequences. Flash operations
aren't supported yet, either.