Apply simplifications for ++ when encountering List.singleton

[jfmengels]

Currently, the following simplification is applied:

[ 1 ] ++ [ 2 ]
--> [ 1, 2 ]

and there are more and more checks where we consider [ 1 ] the same as List.singleton 1. But we don't support the following simplification:

List.singleton 1 ++ [ 2 ]
--> [ 1, 2 ]

[ 1 ] ++ List.singleton 2
--> [ 1, 2 ]

I guess we could also do

List.singleton 1 ++ List.singleton 2
--> [ 1, 2 ]

but I could understand some push-back on that.

[jfmengels]

Nevermind, I was comparing with older versions and got confused. All of my examples get fixed. For instance, List.singleton 1 ++ List.singleton 2 gets fixed to 1 :: List.singleton 2.

One thing that feels missing though and I'm thinking we should push it further is 1 :: List.singleton 2 should probably be fixed to [ 1, 2 ], as that is a lot more idiomatic Elm code.