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.
Putain c'est plus rapide. Et pas qu'un peu.

`./array_tos -m rope --loops 1000000 --strlen 10 --ccts 25 NIT_GC_CHOOSER=large` => 1.83 s
`./array_tos -m flatstr --loops 1000000 --strlen 10 --ccts 25 NIT_GC_CHOOSER=large` => 2.35 s
`./array_tos -m flatbuf --loops 1000000 --strlen 10 --ccts 25 NIT_GC_CHOOSER=large` => 1.32 s
`./array_tos -m manual --loops 1000000 --strlen 10 --ccts 25 NIT_GC_CHOOSER=large` => 0.45 s