Merge remote-tracking branch 'lucas/string_integration'

Part of String refactor, adds a new "chars" method on strings, should be
preferred to the old way of calling Collection services on Strings and
Buffers.

diff --cc lib/mnit_linux/sdl.nit
Simple merge

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

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

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