Future_std: replace Stream with Sequence

Core has Sequence since 111.13.00, which is a Stream-like data
structure. Thus we can use this now and remove dependency on cfstream.

Remove `Pipe.read` from signature because wasn’t immediately obvious
how to provide this for Sequence, and we weren’t using it in Biocaml.
use opam's %{jobs}% variable

Related to #6.