C++: implement doByteArrayNonLiteral - create array from expression o…

…f `bytes` type

Expressions of this type are created using:
- omitting the `type:` field in attributes and parse instances
- declaring parameters with `type: bytes`
- using `.as<bytes>` cast

Ghostly fixes test
```
[info]     - cpp_stl:[0 + 1, 5].as<bytes> *** FAILED ***
[info]       scala.NotImplementedError: an implementation is missing
[info]       at scala.Predef$.$qmark$qmark$qmark(Predef.scala:344)
[info]       at io.kaitai.struct.translators.BaseTranslator.doByteArrayNonLiteral(BaseTranslator.scala:179)
[info]       at io.kaitai.struct.translators.BaseTranslator.doByteArrayNonLiteral(BaseTranslator.scala:28)
[info]       at io.kaitai.struct.translators.CommonArraysAndCast.doByteArray(CommonArraysAndCast.scala:85)
[info]       at io.kaitai.struct.translators.CommonArraysAndCast.doCastOrArray(CommonArraysAndCast.scala:62)
[info]       at io.kaitai.struct.translators.CommonArraysAndCast.doCastOrArray$(CommonArraysAndCast.scala:53)
[info]       at io.kaitai.struct.translators.BaseTranslator.doCastOrArray(BaseTranslator.scala:28)
[info]       at io.kaitai.struct.translators.BaseTranslator.translate(BaseTranslator.scala:147)
[info]       at io.kaitai.struct.translators.AbstractTranslator.translate(AbstractTranslator.scala:25)
[info]       at io.kaitai.struct.translators.AbstractTranslator.translate$(AbstractTranslator.scala:25)
[info]       ...
```
(actually, test is failing because option `useListInitializers` in C++ backend is not set in tests
and code generation without it is not unimplemented right now)

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

@@ -117,6 +117,16 @@ class CppTranslator(provider: TypeProvider, importListSrc: CppImportList, import
 
   override def doByteArrayLiteral(arr: Seq[Byte]): String =
     "std::string(\"" + Utils.hexEscapeByteArray(arr) + "\", " + arr.length + ")"
+  override def doByteArrayNonLiteral(values: Seq[Ast.expr]): String = {
+    // It is assumed that every expression produces integer in the range [0; 255]
+    if (config.cppConfig.useListInitializers) {
+      "std::string({" + values.map(value => s"(char)(${translate(value)})").mkString(", ") + "})"
+    } else {
+      // TODO: We need to produce an expression, but this is possible only with initializer lists
+      // or variadic templates (if use a helper function) which both available only since C++11
+      throw new RuntimeException("C++ non-literal arrays are not implemented yet without list initializers")
+    }
+  }
 
   override def numericBinOp(left: Ast.expr, op: Ast.operator, right: Ast.expr) = {
     (detectType(left), detectType(right), op) match {