Add computedFrom convenience functions

[mcmah309]

Tracking issue for: #360

An alternative representation to the current PR could be

(signal1, signal2, ...).computed((value1, value2, ...) => ...)

And add extension methods for each case

[rodydavis]

Could the generic be of type Record and only need to support 1?

[mcmah309]

I'm not sure what you mean. Are you asking if this is possible?

(signal,).computed((value1) => ...);
// or
signal.computed((value1) => ...);

Then yes.

If you are asking if we only would need to implement one extension method that covers all cases, then no unfortunately. We would need one for each case. Probably best to implemented 5 to 10.