Allow query parameters to erase() 'all but'

[zwoop]

The optional boolean flag argument reverses the list semantics (erase all but the listed query parameters). E.g.

do_remap()
{
  borrow url = Client::URL::get();

  url.query.erase({"foo", "bar"}, true);
}

Which will remove all query parameters, except foo and bar. Alternatively, as a convenience, keep() was also added:

do_remap()
{
  borrow url = Client::URL::get();

  url.query.keep({"foo", "bar"});
}

[brbzull0]

[approve ci autest]