Adds List.push and prepend

[gampleman]

These come up as missing in pipelines quite often. I see a lot of code like

tags =
     node.tags
          |> Maybe.withDefault []
          |> (\tags -> tags ++ [ newTag.label ])
          |> Just

which could be a lot nicer with

tags =
     node.tags
          |> Maybe.withDefault []
          |> List.Extra.push newTag.label
          |> Just

[lue-bird]

I find the prepend name confusing as hell in pipelines (what it's supposed to be used in)

[ 0 ] |> List.prepend [ 1 ] --> [ 0, 1 ]
-- "Wait I though it would prepend 1?

I would strongly suggest to instead introduce two functions prependTo and appendTo where the "to" signals it's use in a pipeline and what the "base list" is.

[ 0 ] |> prependTo [ 1 ] --> [ 0, 1 ]
[ 0 ] |> appendTo [ 1 ] --> [ 1, 0 ]

(used by minibill's elm-rope) but I'd also be fine with names like pushList-consList or appendSuffix-appendPrefix and similar.

[gampleman]

Hm, fair point @lue-bird. Although I don't really like functions that are only pipeline optimized. Perhaps we could do something with the l and r suffix? Like appendl and appendr?

[lue-bird]

I don't think they are "only pipeline optimized".
E.g. remainderBy 3 x is still much more obvious than remainder x 3.
So I'd rather write prependTo [ 1 ] [ 0 ] than prepend [ 1 ] [ 0 ] (even though I can't really see many reasons to use prepend rather than append if you already call the function with both lists). Do you agree?

-l/-r IMO have the same problem as foldl/-r etc, it takes a bit of mental overhead to decipher and still is kind of ambiguous which is exactly the kind of issue a more rarely used helper shouldn't have

[gampleman]

@lue-bird ok, I've implemented the above. Are we happy for this to go in?