add merge_each sequence sender adaptor

merge_each is a sequence adaptor that takes a sequence of nested
sequences and merges all the nested values from all the nested
sequences into a single output sequence.

the first error encountered will trigger a stop request for all
active operations. The error is stored and is emitted only after
all the active operations have completed.

If the error was emitted from an item, a new item is emitted
at the end to deliver the stored error.

any nested sequence or nested value that completes with
set_stopped will not cause any other operations to be stopped.
This allows individual nested sequences to be stopped without
breaking the merge of the remaining sequences.