machine: define Serial as the default output

Previously, the machine.UART0 object had two meanings:

  - it was the first UART on the chip
  - it was the default output for println

These two meanings conflict, and resulted in workarounds like:

  - Defining UART0 to refer to the USB-CDC interface (atsamd21,
    atsamd51, nrf52840), even though that clearly isn't an UART.
  - Defining NRF_UART0 to avoid a conflict with UART0 (which was
    redefined as a USB-CDC interface).
  - Defining aliases like UART0 = UART1, which refer to the same
    hardware peripheral (stm32).

This commit changes this to use a new machine.Serial object for the
default serial port. It might refer to the first or second UART
depending on the board, or even to the USB-CDC interface. Also, UART0
now really refers to the first UART on the chip, no longer to a USB-CDC
interface.

The changes in the runtime package are all just search+replace. The
changes in the machine package are a mixture of search+replace and
manual modifications.

This commit does not affect binary size, in fact it doesn't affect the
resulting binary at all.

Makefile

@@ -349,6 +349,8 @@ smoketest:
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=feather-m4-can      examples/caninterrupt
 	@$(MD5SUM) test.hex
+	$(TINYGO) build -size short -o test.hex -target=arduino-nano33      examples/blinky1
+	@$(MD5SUM) test.hex
 	# test pwm
 	$(TINYGO) build -size short -o test.hex -target=itsybitsy-m0        examples/pwm
 	@$(MD5SUM) test.hex