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_feather-m4-can.go

@@ -103,13 +103,15 @@ var (
 )
 
 var (
-	UART1 = UART{
+	UART1  = &_UART1
+	_UART1 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM5_USART_INT,
 		SERCOM: 5,
 	}
 
-	UART2 = UART{
+	UART2  = &_UART2
+	_UART2 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM0_USART_INT,
 		SERCOM: 0,
@@ -117,8 +119,8 @@ var (
 )
 
 func init() {
-	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM5_2, UART1.handleInterrupt)
-	UART2.Interrupt = interrupt.New(sam.IRQ_SERCOM0_2, UART2.handleInterrupt)
+	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM5_2, _UART1.handleInterrupt)
+	UART2.Interrupt = interrupt.New(sam.IRQ_SERCOM0_2, _UART2.handleInterrupt)
 
 	// turn on neopixel
 	D7.Configure(PinConfig{Mode: PinOutput})