Merge: Benchmark strings update

Update on the benchmarks for String, four micro-benchmarks are now available:

* chain_concat: measures the performance of concatenation
* index_bench: measures random access in a String
* iteration_bench: measures the performance of iteration on a string
* substr_bench: measures the performance of substring on a string

The logic for the benches has changed a lot, instead of working locally with the different structures and having a directory with all the variations (more to come in a near future), now a patch directory is included with the benches for Strings, and will work with all the variants one by one on the chosen benchmark.

Pull-Request: #1640
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>