It seems like a trifle, but if the match frequency is high enough, the
allocation+formatting of line numbers (and columns and byte offsets)
starts to matter. We squash that part of the profile in this commit by
doing our own decimal formatting. I speculate that we get a speed-up
from this by avoiding the formatting machinery and also a possible
allocation.
An alternative would be to use the `itoa` crate, and it is indeed
marginally faster in ad hoc benchmarks, but I'm satisfied enough with
this solution.
1659fb9b43