Review/improve use of empty comments

Links.java has an idiom of using an empty comment as the content of `<a id=..></a> but printing a comment puts a newline (i.e. white space) before and after the comment if needed. If we're going to add whitespace we might as well *just* use whitespace and not bother with the comment, which just becomes noise.

One solution might be to use Entity.ZERO_WIDTH_SPACE instead of EMPTY_COMMENT, but an even better solution would be to put the `a` element around palpable content when possible.