arch.arm.ap: add AP register definition.

arch.arm.dp: add DP register definitions, up to DPv2.

arch.arm.jtag: add JTAG-DP IR/DR definitions.

firmware, software: positively verify API compatibility. Before this commit, firmware and software did not check USB protocol compatibility in any way. Although this mostly worked because there were few changes in USB protocol, there were still breaking changes, the last one in commit 5a7d18a6 that caused a silent hang when wrong firmware version was used. The community is larger now and this sort of breakage is no longer acceptable. After this commit, firmware exports an "API level" (functionally similar to a semver major version) and software verifies that its own API level matches that of the firmware. On mismatch, software reloads the firmware, like it would handle a blank device. Each time firmware is changed in an incompatible way (affecting e.g. control requests, FIFO configuration, ...) the API level must be increased. Miscellaneous cleanups: * In device enumeration code, no more than one handle to a device is now kept open at the same time. Although it doesn't matter on Linux, on Windows opening a USB device is an exclusive operation. * GlasgowHardwareDevice.{get_serial_list→enumerate_serials}

software: simplify firmware management. * Move firmware to glasgow/device/firmware.ihex. * Teach GlasgowHardwareDevice to find firmware unassisted. * Remove all code that accepted custom firmware filenames except for `flash --firmware`. There is no loss of functionality; during development, use `make -C firmware load`, which was always a more reliable option anyway.

support.arepl, cli: gracefully handle Ctrl+C in REPL. Ctrl+C is turned into cancellation if the REPL is running user code, and into KeyboardInterrupt on the main thread (i.e. the default Python behavior) if the REPL is waiting for input.

support.asignal: restore handler if future is cancelled.

support.{pyrepl→arepl}: rewrite with native async/await. As a part of the rewrite, inline code from code.InteractiveConsole and asyncio.AsyncIOInteractiveConsole as necessary. This seems like it would make things more complicated, but it turns out that the code in InteractiveConsole is very old and has too much indirection, which AsyncIOInteractiveConsole painstakingly works around.

platform.rev_c1: explain difference from platform.rev_c0. NFC.

applet.interface.ps2_host: convert to nmigen