Bench string

Added a few benches for variations of String and Array.
Also added a array_debug module on the same idea as hash_debug (PR #664) to track the use of to_s in Array (other functions may be supported someday if needed).

The UTF-8 benchmarks only work with the no-index version for now.
Ça ressortirait mieux avec des backquotes autour de `Array::to_s`.