Remodel pipe support for projections after P2387

[Morwenn]

P2387 describes the different strategies to implement operator| for range adaptors. The support for operator| for projections in cpp-sort is already extremely similar and could certainly benefit from the more mature solution proposed in the paper.

The simplest way to change the design is probably a clean breaking change, capitalizing on the fact that cpp-sort 2.0 is on tracks and will eventually be released.