ksc cpp_stl error: an implementation is missing AnyType (of class io.kaitai.struct.datatype.DataType$AnyType$)
Hi,

This is my first time playing with kaitai struct so maybe I did something wrong but given the following `.ksy` file: 

```yaml
meta:
  id: packet
  file-extension: packet
  endian: be
  encoding: UTF-8
  imports:
    - vlq_base128_le
seq:
  - id: header
    type: header
  - id: message
    type: message
  - id: checksum
    type: u4
types:
  header:
    seq:
      - id: type
        type: u1
      - id: length
        type: u4le
      - id: zero
        type: u1
  message:
    seq:
      - id: op
        type: u4
      - id: id
        type: u8
      - id: body_length
        type: vlq_base128_le
      - id: element_count
        type: vlq_base128_le
      - id: zero
        type: u1
      - id: elements
        type: element
        repeat: expr
        repeat-expr: element_count.value
  element:
    seq:
      - id: type
        type: u1
        enum: element_type
      - id: value
        type:
          switch-on: type
          cases:
            'element_type::byte': u1
            'element_type::short': u2
            'element_type::int': u4
            'element_type::long': u8
            'element_type::float': f4
            'element_type::string': element_string
            'element_type::bin': element_bin
  element_string:
    seq:
      - id: length
        type: u2
      - id: value
        type: str
        size: length
  element_bin:
    seq:
      - id: length
        type: u2
      - id: value
        size: length
enums:
  element_type:
    1: byte
    2: short
    3: int
    4: long
    5: float
    6: string
    7: bin
```
and compiling it like `kaitai-struct-compiler -t cpp_stl .\packet.ksy --debug` gives me the error message:

```
an implementation is missing
AnyType (of class io.kaitai.struct.datatype.DataType$AnyType$)
```

Here's the output with `--ksc-exceptions`:

```
scala.MatchError: AnyType (of class io.kaitai.struct.datatype.DataType$AnyType$)
        at io.kaitai.struct.languages.CppCompiler.kaitaiType2NativeType(CppCompiler.scala:799)
        at io.kaitai.struct.languages.CppCompiler.kaitaiType2NativeType(CppCompiler.scala:806)
        at io.kaitai.struct.languages.CppCompiler.attributeDeclaration(CppCompiler.scala:235)
        at io.kaitai.struct.ClassCompiler.$anonfun$compileAttrDeclarations$1(ClassCompiler.scala:147)
        at io.kaitai.struct.ClassCompiler.$anonfun$compileAttrDeclarations$1$adapted(ClassCompiler.scala:141)
        at scala.collection.immutable.List.foreach(List.scala:389)
        at io.kaitai.struct.ClassCompiler.compileAttrDeclarations(ClassCompiler.scala:141)
        at io.kaitai.struct.ClassCompiler.compileClass(ClassCompiler.scala:87)
        at io.kaitai.struct.ClassCompiler.$anonfun$compileSubclasses$1(ClassCompiler.scala:239)
        at io.kaitai.struct.ClassCompiler.$anonfun$compileSubclasses$1$adapted(ClassCompiler.scala:239)
        at scala.collection.immutable.HashMap$HashMap1.foreach(HashMap.scala:231)
        at scala.collection.immutable.HashMap$HashTrieMap.foreach(HashMap.scala:462)
        at io.kaitai.struct.ClassCompiler.compileSubclasses(ClassCompiler.scala:239)
        at io.kaitai.struct.ClassCompiler.compileClass(ClassCompiler.scala:78)
        at io.kaitai.struct.ClassCompiler.compile(ClassCompiler.scala:25)
        at io.kaitai.struct.Main$.compile(Main.scala:65)
        at io.kaitai.struct.JavaMain.compileSpecAndWriteToFile(JavaMain.scala:282)
        at io.kaitai.struct.JavaMain.$anonfun$compileOneLang$2(JavaMain.scala:265)
        at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
        at scala.collection.mutable.HashMap.$anonfun$foreach$1(HashMap.scala:138)
        at scala.collection.mutable.HashTable.foreachEntry(HashTable.scala:236)
        at scala.collection.mutable.HashTable.foreachEntry$(HashTable.scala:229)
        at scala.collection.mutable.HashMap.foreachEntry(HashMap.scala:40)
        at scala.collection.mutable.HashMap.foreach(HashMap.scala:138)
        at scala.collection.TraversableLike.map(TraversableLike.scala:234)
        at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
        at scala.collection.AbstractTraversable.map(Traversable.scala:104)
        at io.kaitai.struct.JavaMain.compileOneLang(JavaMain.scala:263)
        at io.kaitai.struct.JavaMain.compileOneInput(JavaMain.scala:242)
        at io.kaitai.struct.JavaMain.$anonfun$run$1(JavaMain.scala:212)
        at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
        at scala.collection.immutable.List.foreach(List.scala:389)
        at scala.collection.TraversableLike.map(TraversableLike.scala:234)
        at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
        at scala.collection.immutable.List.map(List.scala:295)
        at io.kaitai.struct.JavaMain.run(JavaMain.scala:210)
        at io.kaitai.struct.JavaMain$.main(JavaMain.scala:201)
        at io.kaitai.struct.JavaMain.main(JavaMain.scala)
AnyType (of class io.kaitai.struct.datatype.DataType$AnyType$)
```

Any suggestions?

Thanks!