Merge: Text API

Merge: Text API

This PR aims to make the `Text` superclass more usable.

Some methods that were only present in `String` have been moved to `Text`, where they keep their copy semantics.
This allows any client to concatenate or multiply any `Text` object.

Also, some convenience methods have been added to `Buffer`, like `insert_at` or `remove_at`.
Note that these methods ~~can~~ **will** be slow since they modify everything in-place and cause shifting the contents of a `Buffer`.

Also, the `RopeBuffer` experimental class was removed from lib since it is not faster than a regular `FlatBuffer` and added extra burden when making changes to the `Text` block.

Pull-Request: #2223
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>