Optimize `Llio`

See https://github.com/Incubaid/arakoon/commit/224dbc9fee05bfd4600d7c965dca61168a423a82.patch for a rationale.

Will launch CI first.
I think we can avoid allocating this string every time this function gets called.
There could be 1 target string used by both int32_to and int64_to. (Not sure if that target string could be shared by input/output_int32/64, as those are lwt-ey...)
The gain from this change is probably marginal though.