[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/AnyUniqueAliases.go

@@ -2,18 +2,34 @@
 
 package Example
 
-type AnyUniqueAliases = byte
+import "strconv"
+
+type AnyUniqueAliases byte
+
 const (
 	AnyUniqueAliasesNONE AnyUniqueAliases = 0
-	AnyUniqueAliasesM AnyUniqueAliases = 1
-	AnyUniqueAliasesT AnyUniqueAliases = 2
-	AnyUniqueAliasesM2 AnyUniqueAliases = 3
+	AnyUniqueAliasesM    AnyUniqueAliases = 1
+	AnyUniqueAliasesT    AnyUniqueAliases = 2
+	AnyUniqueAliasesM2   AnyUniqueAliases = 3
 )
 
 var EnumNamesAnyUniqueAliases = map[AnyUniqueAliases]string{
-	AnyUniqueAliasesNONE:"NONE",
-	AnyUniqueAliasesM:"M",
-	AnyUniqueAliasesT:"T",
-	AnyUniqueAliasesM2:"M2",
+	AnyUniqueAliasesNONE: "NONE",
+	AnyUniqueAliasesM:    "M",
+	AnyUniqueAliasesT:    "T",
+	AnyUniqueAliasesM2:   "M2",
 }
 
+var EnumValuesAnyUniqueAliases = map[string]AnyUniqueAliases{
+	"NONE": AnyUniqueAliasesNONE,
+	"M":    AnyUniqueAliasesM,
+	"T":    AnyUniqueAliasesT,
+	"M2":   AnyUniqueAliasesM2,
+}
+
+func (v AnyUniqueAliases) String() string {
+	if s, ok := EnumNamesAnyUniqueAliases[v]; ok {
+		return s
+	}
+	return "AnyUniqueAliases(" + strconv.FormatInt(int64(v), 10) + ")"
+}