[Go] Make enums into real types, add String() (#5235)

* [Go] Make enums into real types, add String()

This changes the generated code for enums: instead of type aliases,
they're now distinct types, allowing for better type-checking. Some
client code may have to be changed to add casts.

Enum types now have a String() method, so they implement fmt.Stringer.

An EnumValues map is now generated, in addition to the existing
EnumNames map, to easily map strings to values.

Generated enum files are now gofmt-clean.

Fixes #5207

* use example.ColorGreen explicitly

* use valid enum value in mutation test, add new test for "invalid" enum

* add length check and comment

tests/MyGame/Example/Color.go

@@ -2,16 +2,31 @@
 
 package Example
 
-type Color = byte
+import "strconv"
+
+type Color byte
+
 const (
-	ColorRed Color = 1
+	ColorRed   Color = 1
 	ColorGreen Color = 2
-	ColorBlue Color = 8
+	ColorBlue  Color = 8
 )
 
 var EnumNamesColor = map[Color]string{
-	ColorRed:"Red",
-	ColorGreen:"Green",
-	ColorBlue:"Blue",
+	ColorRed:   "Red",
+	ColorGreen: "Green",
+	ColorBlue:  "Blue",
 }
 
+var EnumValuesColor = map[string]Color{
+	"Red":   ColorRed,
+	"Green": ColorGreen,
+	"Blue":  ColorBlue,
+}
+
+func (v Color) String() string {
+	if s, ok := EnumNamesColor[v]; ok {
+		return s
+	}
+	return "Color(" + strconv.FormatInt(int64(v), 10) + ")"
+}