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 while syntax is 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 <jean@pryen.org>