Consider making Reader::lines() reuse one buffer in place #15436
Description
It doesn't make a lot of sense to keep allocating over and over if the app isn't going to hold onto the strings. It hurt our perf vs. Go in https://gist.github.com/alco/5d18c046094d0abb3f58#file-timings
If the user wants new strings he or she can just convert the &str returned into a String.