Merge: String memory fixes

Another step towards the closure of #1106 and #69, this PR removes several calls to `malloc` in `lib/standard` C code and replaces them by some of the solutions proposed in #1106 to handle the allocation of Strings.

@xymus: Your review is more than welcome since we're dealing with extern code, it might be a good time to specify and document how to properly allocate extern data structures to avoid leaking code such as the ones we had.

Pull-Request: #1140
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/standard/string.nit
Simple merge