Thoughts on automatic loop detection? (note from previous PR)

Also I've noticed that some implementations detect an entity loop immediately instead of processing until the recursion limit is reached. Should that be two separate errors in your opinion, or one error?

Obviously it would not need to be added in this PR. Just, as a general concept, should an issue be filed for it

Originally posted by @dralley in #943 (comment)

The problem, is that loop detection should be applied to any entity expansion, and so should be highly coupled with the EntityResolver trait.