Change Set#delete to return Bool

[j8r]

It adds a simple and efficient way to know if the value was present or not.

Design explanations

Having Set#delete returns object or nil is consistent with other #delete methods in the stdlib:

• Hash#delete(object) returns the value if object was present.
• Array#delete_at(index) returns the removed value at the given index, or raises an exception.

However, Deque#delete(object) returns a boolean (maybe it would be better to return a count of deleted objects, but not the topic).

Other options are:

• add Set#delete?(object) : Bool to have parity with Set#add?, but it can be considered redundant
• have Set#delete(object) return a Bool. Why not, but could be confusing: #add does not return a Bool (but #add? does)

Side note: I don't think Set#add and Set#add? are redundant, because Set#<< is using Set#add, and one can easily chain object additions like set << one << second << third.

Closes #9591

[asterite]

Unless nil is a valid value in the set ;-)

[asterite]

I think we should make delete return Bool, and remove add? and make add return Bool.

But as usual, I think it's better to discuss these things in an issue, not in a PR.

[jhass]

The API is good, the implementation not.

[j8r]

I have changed Set#delete to return Bool. It is not possible with the current Hash API to return the deleted key reference.

[jhass]

Fixed outdated specs wording, looks good otherwise