The `JSON::erase()` method for objects changes key ordering

[jviotti]

Consider this new test case that captures the issue:

TEST(JSON_object, erase_must_not_affect_ordering) {
  sourcemeta::core::JSON document{{"x", sourcemeta::core::JSON{1}},
                                  {"y", sourcemeta::core::JSON{2}},
                                  {"z", sourcemeta::core::JSON{2}}};

  auto before_iterator = document.as_object().begin();
  EXPECT_EQ(before_iterator->first, "x");
  std::advance(before_iterator, 1);
  EXPECT_EQ(before_iterator->first, "y");
  std::advance(before_iterator, 1);
  EXPECT_EQ(before_iterator->first, "z");

  document.erase("x");

  auto after_iterator = document.as_object().begin();
  EXPECT_EQ(after_iterator->first, "y");
  std::advance(after_iterator, 1);
  EXPECT_EQ(after_iterator->first, "z");
}

The problems seems to be that FlatMap will swap elements on removal to avoid moving multiple keys. But changing ordering is problematic for other reasons (like linting)