added generic 8bit shift register (#107)

* shifter: added generic 8bit shift register driver with ShiftPin Pin-compatible GPIO interface

examples/shifter/main.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/shifter"
+)
+
+func main() {
+	buttons := shifter.New(shifter.EIGHT_BITS, machine.BUTTON_LATCH, machine.BUTTON_CLK, machine.BUTTON_OUT)
+	buttons.Configure()
+
+	for {
+		// Slower
+		for i := 0; i < 8; i++ {
+			if buttons.Pins[i].Get() {
+				println("Button", i, "pressed")
+			}
+		}
+
+		// Faster
+		pressed, _ := buttons.Read8Input()
+		if pressed&machine.BUTTON_LEFT_MASK > 0 {
+			println("Button LEFT pressed")
+		}
+		if pressed&machine.BUTTON_UP_MASK > 0 {
+			println("Button UP pressed")
+		}
+		if pressed&machine.BUTTON_DOWN_MASK > 0 {
+			println("Button DOWN pressed")
+		}
+		if pressed&machine.BUTTON_RIGHT_MASK > 0 {
+			println("Button RIGHT pressed")
+		}
+		if pressed&machine.BUTTON_SELECT_MASK > 0 {
+			println("Button SELECT pressed")
+		}
+		if pressed&machine.BUTTON_START_MASK > 0 {
+			println("Button START pressed")
+		}
+		if pressed&machine.BUTTON_A_MASK > 0 {
+			println("Button A pressed")
+		}
+		if pressed&machine.BUTTON_B_MASK > 0 {
+			println("Button B pressed")
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}