[5.0] Potential leakage when using signal of composed properties

[andersio]

Due to the signal lifetime semantics we currently have, signal of composed properties could have a potential to leak if either:

1. It is repeatedly called; or
2. It is called but left unused.

Consider the following snippet:

let mapped = property.map { $0 + 1 }
for _ in 0 ..< 100000 {
    _ = mapped.signal
}

It would create 100000 signals from the property that would live as long as property.

Note that even if we share the signal across invocations, the lifetime issue of such signal still persists, since the current semantics of Signal is purely emitter-controlled and push-driven.

We should revisit this after #3075 concludes. 🎄

[mdiep]

This was resolved by #2959.