Closing this following our policy that we don't keep old wishes open that nobody is actively planning to work on.

Clarification: they are already templates, and I don't think we can easily make
them work by making them special cases of a single, more general template.

However, we can probably factor out the common code with a bit of template
metaprogramming, using Trait classes or something like this.

We split issue344 into more manageable chunks. The main issue branch will soon
be merged and this is a possible follow-up work. I nosied everyone from the
original issue, but feel free to erase yourself from nosy, if you are not
interested.

SegmentedArrayVector and SegmentedVector share most of their code. Maybe
templates offer a way to avoid the code duplication withou a performance hit.