[c++] Add bit query on choice set of value via static method. Issue aeron-io#489.

Author: mjpt777

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -666,6 +666,16 @@ private CharSequence generateChoices(final String bitsetClassName, final List<To
 
  sb.append(String.format(
  "\n" +
+ " static bool %1$s(const %2$s bits)\n" +
+ " {\n" +
+ " return %3$s(bits) & (0x1L << %4$s);\n" +
+ " }\n\n",
+ choiceName,
+ typeName,
+ byteOrderStr,
+ choiceBitPosition));
+
+ sb.append(String.format(
  " bool %1$s() const\n" +
  " {\n" +
  "%2$s" +