Actually...I can't find any place where `signatureMap` is used other than the unit test. Do we actually need it? I'm guessing that originally we thought we would use it to try to remap large chunks of the document that might have moved (as in the original BULD algorithm), but since we can do most stuff by our tag IDs we might no longer need to do that.

For the signature itself, I don't think we need to update it here because the signature currently only uses the tag/tagID and not the attributes, so the one that we calculated when we first created the tag is still good. (We  have to recompute it when closing a non-empty tag in order to account for the children, but this case is handling the end of the start tag, which is either an empty tag, or a tag that we'll end up finding the close tag for later.)