More native code gen functionality.

Allow tables to be mapped to native types directly.  For example, a table
representing a vector3 (eg. table Vec3 { x:float; y:float; z:float; }) can
be mapped to a "mathfu::vec3" native type in NativeTables.  This requires
users to provide Pack and UnPack functions that convert between the
Table and native types.  This is done by adding the "native_type" attribute
to the table definition.

To support user-defined flatbuffers::Pack and flatbuffers::UnPack functions,
support a "native_include" markup that will generate a corresponding

Also add an UnPackTo function which allows users to pass in a pointer to
a NativeTable object into which to UnPack the Table.  The existing UnPack
function is now simply:

  NativeTable* UnPack() {
    NativeTable* obj = new NativeTable();
    Table::UnPackTo(obj);
    return obj;
  }

Finally, allow native types to be given a default value as well which are
set in the NativeTable constructor.  This is done by providing a
"native_default" attribute to the member of a table.

Change-Id: Ic45cb48b0e6d7cfa5734b24819e54aa96d847cfd

src/idl_parser.cpp

@@ -174,7 +174,8 @@ std::string Namespace::GetFullyQualifiedName(const std::string &name,
   TD(Include, 269, "include") \
   TD(Attribute, 270, "attribute") \
   TD(Null, 271, "null") \
-  TD(Service, 272, "rpc_service")
+  TD(Service, 272, "rpc_service") \
+  TD(NativeInclude, 273, "native_include")
 #ifdef __GNUC__
 __extension__  // Stop GCC complaining about trailing comma with -Wpendantic.
 #endif
@@ -432,6 +433,10 @@ CheckedError Parser::Next() {
             token_ = kTokenService;
             return NoError();
           }
+          if (attribute_ == "native_include") {
+            token_ = kTokenNativeInclude;
+            return NoError();
+          }
           // If not, it is a user-defined identifier:
           token_ = kTokenIdentifier;
           return NoError();
@@ -1857,6 +1862,10 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
         (attribute_ == "option" || attribute_ == "syntax" ||
          attribute_ == "package")) {
         ECHECK(ParseProtoDecl());
+    } else if (Is(kTokenNativeInclude)) {
+      NEXT();
+      native_included_files_.emplace_back(attribute_);
+      EXPECT(kTokenStringConstant);
     } else if (Is(kTokenInclude) ||
                (opts.proto_mode &&
                 attribute_ == "import" &&