Allow non-'static lifetimes

[milibopp]

This is currently a fairly significant limitation of this crate. Only types T: 'static are allowed for streams and signals. It should in principle be possible to lift this restriction but it might require some tweaks to the implementation. Also, not all parts of the API can work that way. For instance, one cannot feed a sink non-statics in a detached background thread. But the borrow checker should take care of such limitations.

[Moredread]

What causes this?

[milibopp]

Lots of implementation details rely on that, merely because it was part of the generic bounds for all functions for a long time. I think it might be a viable strategy to relax the 'static bounds on some of methods step by step. Quite possibly a lot of the implementation will have to change to allow for that.