[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/AnyAmbiguousAliases.go

@@ -2,18 +2,34 @@
 
 package Example
 
-type AnyAmbiguousAliases = byte
+import "strconv"
+
+type AnyAmbiguousAliases byte
+
 const (
 	AnyAmbiguousAliasesNONE AnyAmbiguousAliases = 0
-	AnyAmbiguousAliasesM1 AnyAmbiguousAliases = 1
-	AnyAmbiguousAliasesM2 AnyAmbiguousAliases = 2
-	AnyAmbiguousAliasesM3 AnyAmbiguousAliases = 3
+	AnyAmbiguousAliasesM1   AnyAmbiguousAliases = 1
+	AnyAmbiguousAliasesM2   AnyAmbiguousAliases = 2
+	AnyAmbiguousAliasesM3   AnyAmbiguousAliases = 3
 )
 
 var EnumNamesAnyAmbiguousAliases = map[AnyAmbiguousAliases]string{
-	AnyAmbiguousAliasesNONE:"NONE",
-	AnyAmbiguousAliasesM1:"M1",
-	AnyAmbiguousAliasesM2:"M2",
-	AnyAmbiguousAliasesM3:"M3",
+	AnyAmbiguousAliasesNONE: "NONE",
+	AnyAmbiguousAliasesM1:   "M1",
+	AnyAmbiguousAliasesM2:   "M2",
+	AnyAmbiguousAliasesM3:   "M3",
 }
 
+var EnumValuesAnyAmbiguousAliases = map[string]AnyAmbiguousAliases{
+	"NONE": AnyAmbiguousAliasesNONE,
+	"M1":   AnyAmbiguousAliasesM1,
+	"M2":   AnyAmbiguousAliasesM2,
+	"M3":   AnyAmbiguousAliasesM3,
+}
+
+func (v AnyAmbiguousAliases) String() string {
+	if s, ok := EnumNamesAnyAmbiguousAliases[v]; ok {
+		return s
+	}
+	return "AnyAmbiguousAliases(" + strconv.FormatInt(int64(v), 10) + ")"
+}