support self-closing tags

[nemith]

This forks encoding/xml to add support for self closing tags. The patch has been upstreamed as https://go-review.googlesource.com/c/go/+/469495. If that patch or something similar to fix https://github.com/golang/go/issues/21399 has been accepted then we can migrate back to the stdlib copy.

We'll have to figure out how best we want to keep this up to date, but i think for this stage this is fine.