interface: add an abstraction layer for character displays

the characterdisplay package is an abstraction layer for controlling
character displays

also includes refactors to the hd44780 package to fit the
characterdisplay Controller interface

samples/characterdisplay.go

@@ -0,0 +1,45 @@
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"time"
+
+	"github.com/kidoman/embd"
+	"github.com/kidoman/embd/controller/hd44780"
+	"github.com/kidoman/embd/interface/display/characterdisplay"
+
+	_ "github.com/kidoman/embd/host/all"
+)
+
+func main() {
+	flag.Parse()
+
+	if err := embd.InitI2C(); err != nil {
+		panic(err)
+	}
+	defer embd.CloseI2C()
+
+	bus := embd.NewI2CBus(1)
+
+	controller, err := hd44780.NewI2C(
+		bus,
+		0x20,
+		hd44780.PCF8574PinMap,
+		hd44780.RowAddress20Col,
+		hd44780.TwoLine,
+		hd44780.BlinkOn,
+	)
+	if err != nil {
+		panic(err)
+	}
+
+	display := characterdisplay.New(controller, 20, 4)
+	defer display.Close()
+
+	display.Clear()
+	display.Message("Hello, world!\n@embd | characterdisplay")
+	time.Sleep(10 * time.Second)
+	display.BacklightOff()
+}