Fixed Cell memory leak in signaled subscriptions

[wkok]

When using input signals with subscriptions (layer 3 - materialized views) I've noticed gradually degraded responsiveness in a UI that subscribed to many of these form of subscriptions.

Upon investigation, and with the help of the Flutter Dev Tools, I've identified an issue that seemed to point to a memory leak of Cell instances:

And indeed after some debugging, I've confirmed that when re-dash subscriptions were used as input signals, they created new Cell instances each time the signal gets resolved to a new value, and the old Cell was not garbage collected (probably because it remained referenced by its original dependencies)

This is corrected in this PR by introducing a signal cache (a registry if you will, like we use for normal subscriptions registered with reg-sub, but only for subscription signals)

This is managed internally by re-dash (as with all other regsitries) no changes required to existing code.

Result is much better UI responsivenes that does not degrade over time, and Flutter Dev Tools agree:

[hti-andre]

🚀😎