Proposal: add it.FilterSeq2

[andig]

We habe it.Filter which works on a iter.Seq, we have it.FilterKeys returning iter.Seq[K] but we're missing it.FilterSeq2 to return the (obvious?) iter.Seq2[K, V].

Am I missing something?

[samber]

FilterKeys and FilterValues apply to maps not slices.

I think we need to find a norm for Seq/Seq2 variants.

SeqToChannel has SeqToChannel2 variant but i'm not sure it has been named properly. (see #805)

[andig]

I was actually referring to filtering a map into a Seq2, not a slice! But maybe there‘s also little general use for that. My case in the end boiled down to a range loop with an if not continue short route.