function xtab uses sort(keys)

[cgoldammer]

Instead, the function should probably use sort(collect(keys)). Do you want me to put in a PR (including mild refactoring, docstrings, and added test case)?

The only reason I'm asking before just writing the PR is that #48 mentions that you're planning to move xtab and xtabs over to Stats. I can't see why fixing xtab before doing so would hurt, but want to make sure.