Right function for Iterable<Either<E, A>>

[serras]

During the discussion in #2795, the issue of a good function that works over Iterable<Either<E, A> was raised by @pakoito. Here are some of the ideas gathered, so we can discuss them separately from that PR.

1. Iterable<Either<E, A>>.sequence(): Either<E, List<A>>

This function is very similar to the sequence found in other languages; it returns the original list of elements, or bails out if any of them is Left. Note that this only returns the first error.

2. Iterable<Either<E, A>>.flattenOrAccumulate(): Either<NonEmpty<E>, List<A>>

This was suggested by @nomisRev in an internal conversation. The idea is that it works like sequence above, but doesn't stop at the first error. This suits a bit better with the new API in which we try to maintain errors as far as possible, as we used to do with Validated.

3. suspend Iterable<Either<E, A>>.bindAll(): List<A>

This would be just a shortcut to map { it.bind() }. The idea is to stay within the DSL, but has the disadvantage that you need to write either { list.bindAll() } to recover the behavior of sequence; it also stops on the first error.

[serras]

This PR #2872 proposes yet another way to introduce an accumulating function, in this case generic over every Raise.

[nomisRev]

Here is a use-case for the 2nd signature, https://github.com/arrow-kt/arrow/pull/2863/files#diff-74bf2a702683ff9b5377a1d02d00a2708862d90d5d5dc25e72f675283ee14b1cR73. Name is up for debate, but it's needed to derive the parallel version mapOrAccumulate.

After some discussion in the last bi-weekly Arrow meeting, I am in favour of adding both options 2 and 3. IMO bindAll makes more sense than having sequence.