Merge: lib/string: some cleaning, doc and unit tests

An important point is the generalization of services from
StringCharView to Sequences, and the privatization of StringCharView.

Closes #144
Closes #140
Pull-Request: #383
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc lib/standard/collection/abstract_collection.nit
Simple merge

diff --cc lib/standard/collection/array.nit
Simple merge