lib/pipeline: add basic functional-like filer processing (map&select)
lib/pipeline: add basic functional-like filer processing (map&select)

Because Nit is not functional, worse, there is no anonymous classes, the
use is complex.
However, I tried to have the syntax as light as it could be currently.

I still suspect that for most cases, a `for` loop can perform
enough map&select style. Therefore, consider this commit as an
experimentation.

Signed-off-by: Jean Privat <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

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