[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/namespace_test/NamespaceA/NamespaceB/EnumInNestedNS.go

@@ -2,7 +2,10 @@
 
 package NamespaceB
 
-type EnumInNestedNS = int8
+import "strconv"
+
+type EnumInNestedNS int8
+
 const (
 	EnumInNestedNSA EnumInNestedNS = 0
 	EnumInNestedNSB EnumInNestedNS = 1
@@ -10,8 +13,20 @@ const (
 )
 
 var EnumNamesEnumInNestedNS = map[EnumInNestedNS]string{
-	EnumInNestedNSA:"A",
-	EnumInNestedNSB:"B",
-	EnumInNestedNSC:"C",
+	EnumInNestedNSA: "A",
+	EnumInNestedNSB: "B",
+	EnumInNestedNSC: "C",
 }
 
+var EnumValuesEnumInNestedNS = map[string]EnumInNestedNS{
+	"A": EnumInNestedNSA,
+	"B": EnumInNestedNSB,
+	"C": EnumInNestedNSC,
+}
+
+func (v EnumInNestedNS) String() string {
+	if s, ok := EnumNamesEnumInNestedNS[v]; ok {
+		return s
+	}
+	return "EnumInNestedNS(" + strconv.FormatInt(int64(v), 10) + ")"
+}