list-ops: simplify generics

[senekor]

closes #2053

[Morgane55440]

i'd like to also mentions that the changes i proposed slightly change only the first function append

the original append simplified would look like

pub fn append<I : Iterator>(fst: I, snd: I) -> impl Iterator<Item = I::Item>

but i wrote it

pub fn append<I : Iterator, J>(fst: I, snd: J) -> impl Iterator<Item = I::Item>
where
    J : Iterator<Item = I::Item>

because it feels reasonable that it should support different types of input, if you feel it's not worth changing feel free to use the first solution.

Everything else is the exact same, just written differently

[senekor]

Yeah I saw that and liked the additional flexibility, but it actually seems to break type inference (not sure why). I'll to figure it out and otherwise revert that particular change.

Edit: just a silly mistake, I wrote <I, J>(_a: I, _b: I) (b shoudl be J not I) so the parameter J was unused.

[ellnix]

because it feels reasonable that it should support different types of input

I really like this as well.

The changes are very good overall, I can't see a problem with them. I think it would be appropriate to add both @Morgane55440 and @senekor as authors or contributors in config.json, since this change is sizable and @senekor came up with the idea of using generics in the first place.