Merge: Bench strings update

A few things happening here:

* Added some line-producing output to `bench_plot.sh`, use both functions to produce lines instead of histograms
* `bench_strings.sh` underwent a few transformations: now you can try with different values of `maxlen` (threshold at which a flat concatenation produces a new Concat node instead of a new FlatString), you can also work on HEAD only with a new option, also, all benches now produce lines instead of histograms

Pull-Request: #1795
Reviewed-by: Jean Privat <jean@pryen.org>