Unify named character reference tokenizer of xml5ever and html5ever

[simonwuelker]

46a9b0a#diff-4969d7c75c41dbd134d41920b1ae213f6f7169044659add391e38f32ab156cde pretty much copied the character reference tokenizer from html5ever to xml5ever. We could put it in a shared crate (like markup5ever) instead.

This would be useful for #645.