Fix for #1220

I know it's late in the release process and all, but ...
in case there should be a 2.8.0.RC7 - would it be
possible to include a fix for #1220 (Streams not being
thread-safe)?

Since such a fix, while simple, would mean changing
Stream.cons, it will break binary compatibility.
So it might be better to do it now, instead of in
a minor release later on.

Also, having a non-thread safe, immutable Stream
seems a bit strange - it will prevent many people
who know about this from using it, and cause mean
bugs for people who don't (it's not documented and
one would usually assume that immutable objects can
safely be shared between threads).

I know it's an old bug, but recent cc's seem to indicate
an active interest.

Cheers,

Oliver