List API is inconsistent with other modules

[cknitt]

This is for historical reasons (the List module is copied from Belt).

For example:

• List: let forEach: (t<'a>, 'a => 'b) => unit
• Others: let forEach: (t<'a>, 'a => unit) => unit

or

• List: let mapWithIndex: (t<'a>, (int, 'a) => 'b) => t<'b>
• Others: let mapWithIndex: (t<'a>, ('a, int) => 'b) => t<'b>

or

• List: getBy
• Others: find

[zth]

@cknitt I think we can change it to make it consistent. Good to do before Core goes into the compiler. What do you think?

[cknitt]

Yes, absolutely, I agree we should make it consistent before Core goes into the compiler.
I can make a PR for that some time in the next days.

[cknitt]

@zth See #195.

But there are still some open questions:

1. List has type t<'a> = list<'a>. Array doesn't have a t<'a> defined, and t<'a> was also removed from Result recently. If the idea is to define a t<'a> only where necessary, then it should be removed from List, too?
2. List has toShuffled, but sort, reverse etc. all without to. As this is an immutable data structure, we do not need the distinction between shuffle and toShuffled like in Array. So remove the to?
3. List.concatMany signature is different from Array.concatMany. Personally I find the Array.concatMany signature weird anyway (and different from what it was in Belt.Array) and would actually change it there.
4. getAssoc, hasAssoc, removeAssoc, setAssoc: These serve for abusing a list as a map. Given that we have both the mutable JS Map and the immutable Belt.Map available in ReScript, should we really keep these?
5. Map and Set have has and size, Array has includes and length. What should List have?

[cknitt]

Personally I find the Array.concatMany signature weird anyway (and different from what it was in Belt.Array) and would actually change it there.

Which would then be the same as Array.flat. Hmmm. So not sure what to do about the different concatManys.