dtoh: Provide operator[] for _d_dynamicArray ...

... s.t. it can be used like a normal array (+ bounds checking)

test/dshell/extra-files/cpp_header_gen/app.cpp

@@ -13,6 +13,8 @@ int main()
     assert(!c->s.b);
     assert(c->name.ptr == name);
     assert(c->name.length == length);
+    assert(c->name[1] == 'e');
+    assert(const_cast<const C*>(c)->name[2] == 'a');
     c->verify();
 
     assert(foo(c->s) == bar(c));