[FS-1042] Implement transpose on Seq, List and Array

[PatrickMcDonald]

Implements RFC https://github.com/fsharp/fslang-design/blob/master/FSharp.Core-4.4.3.0/FS-1042-add-seq-transpose.md

[abelbraaksma]

I haven't checked the code, but I think this is a valuable addition to the library!

[dsyme]

If the first inner.MoveNext is false then inner never gets added to rm and thus never gets disposed

[dsyme]

I feel something is odd about this whole implementation. If I do a Seq.toArray on the overall result, without iterating each individual sequence, then none of the inner enumerators have been created, and rm will not have been populated. But we will have already executed the outer finally` designed to cleanup the rm`` array. This feels wrong. I suspect you may just have to create a strict data structure and return simpler iterators.

[PatrickMcDonald]

All good points, this implementation was kind of a brain dump, while trying not to eagerly evaluate any sequences. I'll take a look at it later on and try and fix.

[PatrickMcDonald]

So I've been playing around with Seq.transpose, something like the following works, but not for infinite lists

    [<CompiledName("Transpose")>]
    let transpose (source: seq<seq<'T>>) =
        checkNonNull "source" source
        source
        |> collect indexed
        |> groupBy fst
        |> map (snd >> (map snd))

If we were to allow one dimension of the source to be infinite, which would it be?
n x inf or inf x n

[dsyme]

@PatrickMcDonald The above is very natural, I would go with that, at least on first look

[gusty]

I think it should be possible to make it work with both infinite dimensions, however it will require maintaining a buffer of enumerators, but I would be an interesting feature.

[KevinRansom]

Thanks for this.

@ dsyme ... you good with this. For 4.3?