Improve Equatable implementation of SubscriptionEffect

[pteasima]

With number of init overloads mounting up, the way I do equtability + type erasure is getting annoying.

https://github.com/pteasima/Uniflo/blob/master/Sources/Uniflo/SubscriptionEffect.swift

I should really look into AnyEquatable, but atm I dont really understand it and dont want to spend time on it.

PRs very much appreciated.

[pteasima]

could also use some more codereuse between the different overloads in general. Atm Im very mechanically copypasting cause getting reuse right for all three of perform,data and eq isnt straight forward.