Generate enums as interface + enum + class for unknown values

The new generated enum is represented as an interface with two nested classes:
- enum class Known with the list of known enumeration values
- class Unknown which represents any not-known value

The enums `enum` would be generated as:

```java
interface Enum extends IKaitaiEnum {
    public static class Unknown extends IKaitaiEnum.Unknown implements Enum {
        private Unknown(long id) { super(id); }
    }
    public enum Known implements Enum {
        Variant1(1),
        Variant2(2),
        Variant(3);

        private final long id;

        static HashMap<Long, Enum> variants = new HashMap<>(3);
        static {
            for (final Known e : Known.values()) {
                variants.put(e.id, e);
            }
        }

        private Known(long id) { this.id = id; }
        @OverRide
        public long id() { return this.id; }
    }

    public static Enum byId(final long id) {
        return Known.variants.computeIfAbsent(id, _id -> new Unknown(id));
    }
}
```

Unfortunately, it is not possible to generate enum what will implement nested interface
due to cyclic reference. If that would be possible, we would be protected against name clashes.
In the current implementation the names Known and Unknown can clash with enum name itself

shared/src/main/scala/io/kaitai/struct/languages/JavaCompiler.scala

@@ -690,7 +690,15 @@ class JavaCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
     val enumClass = type2class(enumName)
 
     out.puts
-    out.puts(s"public enum $enumClass {")
+    out.puts(s"public interface $enumClass implements IKaitaiEnum {")
+    out.inc
+    out.puts(s"public static class Unknown extends IKaitaiEnum.Unknown implements $enumClass {")
+    out.inc
+    out.puts("Unknown(long id) { super(id); }")
+    out.dec
+    out.puts("}")
+    out.puts
+    out.puts(s"public enum Known implements $enumClass {")
     out.inc
 
     if (enumColl.size > 1) {
@@ -705,23 +713,37 @@ class JavaCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
 
     out.puts
     out.puts("private final long id;")
-    out.puts(s"$enumClass(long id) { this.id = id; }")
-    out.puts("public long id() { return id; }")
-    out.puts(s"private static final Map<Long, $enumClass> byId = new HashMap<Long, $enumClass>(${enumColl.size});")
+    out.puts(s"static final HashMap<Long, $enumClass> variants = new HashMap<>(${enumColl.size});")
     out.puts("static {")
     out.inc
-    out.puts(s"for ($enumClass e : $enumClass.values())")
+    out.puts(s"for ($enumClass e : $enumClass.values()) {")
     out.inc
-    out.puts(s"byId.put(e.id(), e);")
+    out.puts(s"variants.put(e.id(), e);")
+    out.dec
+    out.puts("}")
+    out.dec
+    out.puts("}")
+    out.puts
+    out.puts("Known(long id) { this.id = id; }")
+    out.puts
+    out.puts("@Override")
+    out.puts("public long id() { return id; }")
+    out.puts
+    out.puts("@Override");
+    out.puts(s"public String toString() { return \"${$enumClass}(\" + this.id + \")>\"; }");
     out.dec
+    out.puts("}")
+    out.puts
+    out.puts(s"public static $enumClass byId(final long id) {")
+    out.inc
+    out.puts("return Known.variants.computeIfAbsent(id, _id -> new Unknown(id));")
     out.dec
     out.puts("}")
-    out.puts(s"public static $enumClass byId(long id) { return byId.get(id); }")
     out.dec
     out.puts("}")
 
-    importList.add("java.util.Map")
     importList.add("java.util.HashMap")
+    importList.add("io.kaitai.struct.IKaitaiEnum")
   }
 
   override def debugClassSequence(seq: List[AttrSpec]) = {

shared/src/main/scala/io/kaitai/struct/translators/JavaTranslator.scala

@@ -67,7 +67,7 @@ class JavaTranslator(provider: TypeProvider, importList: ImportList) extends Bas
     s"${JavaCompiler.publicMemberName(id)}()"
 
   override def doEnumByLabel(enumTypeAbs: List[String], label: String): String =
-    s"${enumClass(enumTypeAbs)}.${Utils.upperUnderscoreCase(label)}"
+    s"${enumClass(enumTypeAbs)}.Known.${Utils.upperUnderscoreCase(label)}"
   override def doEnumById(enumTypeAbs: List[String], id: String): String =
     s"${enumClass(enumTypeAbs)}.byId($id)"