[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/Any.go

@@ -2,18 +2,34 @@
 
 package Example
 
-type Any = byte
+import "strconv"
+
+type Any byte
+
 const (
-	AnyNONE Any = 0
-	AnyMonster Any = 1
+	AnyNONE                    Any = 0
+	AnyMonster                 Any = 1
 	AnyTestSimpleTableWithEnum Any = 2
 	AnyMyGame_Example2_Monster Any = 3
 )
 
 var EnumNamesAny = map[Any]string{
-	AnyNONE:"NONE",
-	AnyMonster:"Monster",
-	AnyTestSimpleTableWithEnum:"TestSimpleTableWithEnum",
-	AnyMyGame_Example2_Monster:"MyGame_Example2_Monster",
+	AnyNONE:                    "NONE",
+	AnyMonster:                 "Monster",
+	AnyTestSimpleTableWithEnum: "TestSimpleTableWithEnum",
+	AnyMyGame_Example2_Monster: "MyGame_Example2_Monster",
 }
 
+var EnumValuesAny = map[string]Any{
+	"NONE":                    AnyNONE,
+	"Monster":                 AnyMonster,
+	"TestSimpleTableWithEnum": AnyTestSimpleTableWithEnum,
+	"MyGame_Example2_Monster": AnyMyGame_Example2_Monster,
+}
+
+func (v Any) String() string {
+	if s, ok := EnumNamesAny[v]; ok {
+		return s
+	}
+	return "Any(" + strconv.FormatInt(int64(v), 10) + ")"
+}