Patch the standard library to make use of concepts like Iterable and Indexable

[Araq]

Once the most pressing concept bugs are fixed, the stdlib should make use of them. Especially the concepts Iterable and Indexable should be introduced and used in the standard library, in particular in the modules sequtils and algorithm for a start.

Suggestion:

type
  Iterable* = concept c
    for x in items(c): discard

  Indexable*[T] = concept c
    var i: int
    c[i] is T
    c.len is int

[awr1]

Should we add a concept for seq-ish stretchy buffer types as well? e.g. supports add(), setLen()? I recall years ago someone implementing their own alternative to seq in Nim - not really sure what the reason was. Such an addition is probably super unnecessary but just throwing that out there.

Deferring to @mratsim 's suggestion here I would suggest sequtils.map() should return the same type as its input (and presumably change the input type to one that satisfies Iterable). It might also be ideal to change the name of sequtils to something like containers or higherorder or functors.

[timotheecour]

• with today's compiler, we can't use Iterable concept for something that accepts an iterator such as toSeq(myIter(3)), see https://github.com/nim-lang/Nim/issues/8774#issuecomment-415993902
  I have an idea for how to fix this (in which case Iterable concept could be used), but haven't submitted RFC yet (@zah also had an idea (see https://github.com/nim-lang/Nim/issues/8774#issuecomment-416022899) maybe it's the same)

• I would suggest sequtils.map() should return the same type as its input

IMO the best is instead for map + friends to return an iterator; that view is shared by @zah here

I do share the view that the best possible way to define map is as an iterator [see rest of comment]

that should be done in a different module to avoid confusion; RFC to come later.

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[ZoomRmc]

Since now we have a iterable[t] that's a magic type, it would be nice to have some comparison between the two options and plans for defining the proper way of handling iterators.