Add core support for multiple UARTs (#152)

* machine/uart: add core support for multiple UARTs by allowing for multiple RingBuffers
* machine/uart: complete core support for multiple UARTs
* machine/uart: no need to store pointer to UART, better to treat like I2C and SPI
* machine/uart: increase ring buffer size to 128 bytes
* machine/uart: improve godocs comments and use comma-ok idiom for buffer Put/Get methods

src/machine/machine_nrf.go

@@ -54,10 +54,15 @@ func (p GPIO) Get() bool {
 	return (port.IN>>pin)&1 != 0
 }
 
+// UART on the NRF.
+type UART struct {
+	Buffer *RingBuffer
+}
+
 // UART
 var (
 	// UART0 is the hardware serial port on the NRF.
-	UART0 = &UART{}
+	UART0 = UART{Buffer: NewRingBuffer()}
 )
 
 // Configure the UART.
@@ -108,7 +113,7 @@ func (uart UART) WriteByte(c byte) error {
 
 func (uart UART) handleInterrupt() {
 	if nrf.UART0.EVENTS_RXDRDY != 0 {
-		bufferPut(byte(nrf.UART0.RXD))
+		uart.Receive(byte(nrf.UART0.RXD))
 		nrf.UART0.EVENTS_RXDRDY = 0x0
 	}
 }