Add optimization that fuses together map functions

[jfmengels]

The idea of this optimization is to avoid having consecutive map functions, and instead grouping them using composition. So instead of:

x |> List.map f1 |> List.map f2

we'd in practice end up with

x |> List.map (f1 >> f2)

Some benchmarks can be found in this folder: https://github.com/jfmengels/elm-benchmarks/tree/master/src/FusionExploration
It seems to be worthwhile to have, as shown for instance by this benchmark:

Function composition like this has its own performance drawbacks compared to lambdas, but that's where #73 can come in and save the day.

I made this branch a long time ago, and I don't remember where it's at, or why I didn't end up creating a PR for it. So I'll just make a draft and then other people can see it or take inspiration from it. Ping me to put eyes on it again when some time gets unblocked for this project.

[kutyel]

Very nice to see the performance benchmarks, thanks for this! 💪🏻