Add a default "comments" feature to transform comments into errors when
unset.
This is so that XMPP implementations don’t have to care about comments,
as they can’t happen in the stream.
quick_xml is way faster than xml-rs
Here is an example with a quick atom parser:
With xml-rs:
test parse_factorio_atom ... bench: 3,295,678 ns/iter (+/- 165,851)
With quick_xml:
test parse_factorio_atom ... bench: 203,215 ns/iter (+/- 13,485)
Unfortunately I had to break the API for this change to happen.
* Element::from_reader now takes `R: BufRead` instead of `R: Read`
* Element::write_to now takes `W: io::Write` instead of `EventWriter<W: Write>`
This migration also allow us to have a write_to function which assumes
we're already in a given namespace (see `write_to_in_namespace`).
