AllSatisfy operator

[vitallica]

• Added an AllSatisfy operator.
• Added tests.
• Updated README.

This is my first contribution to this library, so please feel free to offer any feedback.

[freak4pc]

Hey @vitallica ... It seems this operator already exists?
https://developer.apple.com/documentation/combine/publishers/allsatisfy

[vitallica]

Hey @freak4pc, thanks for looking at this PR.

The native operator does not handle situations where Output is of type Collection, since the predicate takes a value of type Self.Output. This operator has a predicate that takes a value of type Output.Element.

[vitallica]

@freak4pc while investigating further, I found a case in which the compiler chooses the native operator rather than this one.

let names = ["John", "Jane", "Jim", "Jill", "Joan"]

Just(names)
  .allSatisfy { $0.count <= 4 }
  .sink { print("all short? \($0)") }

This emits false, because the array itself has a count of 5. However it's possible to force the overload like so:

Just(names)
  .allSatisfy { (name: String) in name.count <= 4 }
  .sink { print("all short? \($0)") }

This emits true, because all of the elements within the array have count <= 4.

In order to avoid this conflict, I think I should rename this operator to AllSatisfyMany to match the naming convention of CombineLatestMany, MapMany, and MergeMany. What do you think?