Allow non-Clone types

[milibopp]

Currently all values passed into a sink to be processed by the event graph, have to satisfy quite a number of trait bounds, namely Send + Sync + Clone. Passing an object down the event graph is achieved by cloning. This is not optimal for larger heap-allocated types, say a Vec or a HashMap, where cloning is expensive. This can be alleviated by wrapping those types in an Arc, so that only references will be sent down the event graph. However, this is a bit clumsy to work with.

Using Arc

As an alternative we could use Arc internally for all values. Functions passed to the primitives would then have to take their arguments by reference and similarly .sample() and .events() would yield references.

On the upside, working with cells and streams would become more ergonomic, as one would not have to think about expensive cloning of each event sent into a stream. It also drops the Clone bound on the type of an event.

The downside is, of course, that this constitutes a bit of an unnecessary overhead for small types. But then again, the library currently uses a lot of atomic ref-counting, vtable dispatch and pointer indirection. One more indirection would likely not be that harmful.

[milibopp]

Negative trait bounds would allow the best of both worlds.

[milibopp]

Having tried this once again, it appears that the internals would have to be much more sophisticated to allow this. It's not clear that this is actually worth it. Thus closing.