Consider turning global/import.ml into a header

[cuihtlauac]

To reduce the mental load when reading the code, highlighting “something” which looks like Stdlib but actually isn't is useful. The module Import could be phased out in three steps:

• Using actual external dependencies instead of blocks of copied code, for instance, get String.cut from the package where it's defined and add it as a dependency
• Move stuff which does not extend actual Stdlib modules elsewhere, maybe with a renaming (e.g. Acc_biggest)
• Turn the rest (it should only be oneliners) into a copied header in all the files where it makes sense. That way, understanding remains local, but algorithms which are generic are highlighted as such.

[sabine]

Indeed, consolidating all the strewn-around Stdlib extensions into one module was a good first step, now to assimilate and rearrange so we don't need to do much Stdlib extension at all is great!