Merge: String split

Right now, the string module is over 2500 loc long.

For the mental health of the maintainer(s), it was getting too big to understand what was going on in it.

So, this PR introduces the text group in standard, string is split in several modules inside of it:

* native: Introduces NativeString
* abstract_text: Introduction of basic types
* flat: Any array-based string
* ropes: Rope-based strings

Big con: To merge this PR, bootstrap will need to support the nullable arguments syntax.

But in the meantime, please feel free to comment on the code

Pull-Request: #1400
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

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

diff --cc lib/standard/text/abstract_text.nit
Simple merge

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