Merge strings further

[Baptistemontan]

There already was some kind of duplicates strings elimination, this is just a PoC to also eliminate duplicate substrings.
This could be further improved because it is not retroactive, so if abcd is registered after bc, the string will be bcabcd where it could only have been abcd.
The substring resolution is a call to str::find, so it's a linear search (maybe even O(n²) ?) and is called for every strings, so it can be pretty expensive depending on the number of strings. For this reason it might be great to gate it behing a feature.