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/machine_atsamd21.go

@@ -515,7 +515,7 @@ const (
 )
 
 // Configure the UART.
-func (uart UART) Configure(config UARTConfig) error {
+func (uart *UART) Configure(config UARTConfig) error {
 	// Default baud rate to 115200.
 	if config.BaudRate == 0 {
 		config.BaudRate = 115200
@@ -614,7 +614,7 @@ func (uart UART) Configure(config UARTConfig) error {
 }
 
 // SetBaudRate sets the communication speed for the UART.
-func (uart UART) SetBaudRate(br uint32) {
+func (uart *UART) SetBaudRate(br uint32) {
 	// Asynchronous fractional mode (Table 24-2 in datasheet)
 	//   BAUD = fref / (sampleRateValue * fbaud)
 	// (multiply by 8, to calculate fractional piece)
@@ -628,7 +628,7 @@ func (uart UART) SetBaudRate(br uint32) {
 }
 
 // WriteByte writes a byte of data to the UART.
-func (uart UART) WriteByte(c byte) error {
+func (uart *UART) WriteByte(c byte) error {
 	// wait until ready to receive
 	for !uart.Bus.INTFLAG.HasBits(sam.SERCOM_USART_INTFLAG_DRE) {
 	}