machine: make UART objects pointer receivers

This means that machine.UART0, machine.UART1, etc are of type
*machine.UART, not machine.UART. This makes them easier to pass around
and avoids surprises when they are passed around by value while they
should be passed around by reference.

There is a small code size impact in some cases, but it is relatively
minor.

src/machine/board_nucleof722ze.go

@@ -32,17 +32,18 @@ var (
 	// USART3 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
 	// Both UART0 and UART1 refer to USART2.
-	UART0 = UART{
+	UART0  = &_UART0
+	_UART0 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.USART3,
 		TxAltFuncSelector: UART_ALT_FN,
 		RxAltFuncSelector: UART_ALT_FN,
 	}
-	UART1 = &UART0
+	UART1 = UART0
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART3, UART0.handleInterrupt)
+	UART0.Interrupt = interrupt.New(stm32.IRQ_USART3, _UART0.handleInterrupt)
 }
 
 // SPI pins