all: use interfaces for UART objects

This makes it possible to replace UART objects with dummy
implementations, for example. Or allows changing the `machine.UART` type
to `*machine.UART` without breaking compatibility.

uart.go

@@ -0,0 +1,12 @@
+package drivers
+
+import "io"
+
+// UART represents a UART connection. It is implemented by the machine.UART
+// type.
+type UART interface {
+	io.Reader
+	io.Writer
+
+	Buffered() int
+}