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_teensy36.go

@@ -80,11 +80,11 @@ const (
 )
 
 var (
-	TeensyUART1 = &UART0
-	TeensyUART2 = &UART1
-	TeensyUART3 = &UART2
-	TeensyUART4 = &UART3
-	TeensyUART5 = &UART4
+	TeensyUART1 = UART0
+	TeensyUART2 = UART1
+	TeensyUART3 = UART2
+	TeensyUART4 = UART3
+	TeensyUART5 = UART4
 )
 
 const (