Issue 7655: align the bench printouts so that the numbers tend to be aligned.

(scratching an itch.)
auto merge of #9823 : pnkfelix/rust/issue7655-bench-papercuts, r=alexcrichton

r? anyone.

You can see a bit more discussion on #7655.

This does not close any ticket; I am just scratching an itch.  Note in particular that I picked the value `{:>9} ns/iter` pretty much out of a hat.  :)