[ruby-core:98970] [Ruby master Feature#16995] Sets: <=> should be specialized

Issue #16995 has been reported by marcandre (Marc-Andre Lafortune).

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Feature #16995: Sets: <=> should be specialized
https://bugs.ruby-lang.org/issues/16995

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
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This is quite minor, but `Set#<=>` should be refined.

Reminder: `Set` defines `<`, `>`, etc. as inclusion, but does not have a corresponding `<=>`:

```ruby
Set[1] < Set[1, 2] # => true
Set[1] <=> Set[1, 2] # => nil, should be -1
Set[1] <=> Set[2] # => nil, ok, not orderable
```

The official stated reason for `Set` to *not* implement is that some sets are not comparable. That is exactly what `nil` result type is for IMO. Sets are partically ordered and `<=>` should reflect that. https://en.wikipedia.org/wiki/Partially_ordered_set

```ruby
Set[1] < Set[1, 2] # => true
[Set[1], Set[1, 2]].sort # => ArgumentError, should be [Set[1], Set[1, 2]]
[Set[1], Set[2]].sort # => ArgumentError, ok, can't be ordered
```

This is *exactly the same* idea as `Class`, which correctly refines `<=>`:

```ruby
Array < Enumerable # => true
Array <=> Enumerable # => -1, ok
[Array, Enumerable].sort # => [Array, Enumerable]
[Array, String].sort # => ArgumentError (comparison of Class with Class failed), ok
```



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