Merge: introduce plain_to_s

Merge: introduce plain_to_s

In order to have a more POLA `to_s` on collections, it is required that a new method with the old behavior is provided for clients that need it (especially the interpreter that use it for superstrings).

the compiler use `native_to_s` for superstrings but `c_src/nitg` still use `to_s` on array, so a regeneration of the bootstrap is required before changing the behavior of `to_s` on collections.

Pull-Request: #1385
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Blondin Massé <alexandre.blondin.masse@gmail.com>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>