[ruby-core:100259] [Ruby master Feature#17208] Add `Set#compact` and `Set#compact!` methods

Issue #17208 has been reported by koic (Koichi ITO).

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Feature #17208: Add `Set#compact` and `Set#compact!` methods
https://bugs.ruby-lang.org/issues/17208

* Author: koic (Koichi ITO)
* Status: Open
* Priority: Normal
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This is a proposal to add `compact` and `compact!` methods already owned by `Array` and `Hash` to `Set`.

- `Array#compact` and `Array#compact!` has been around for a long time.
- `Hash#compact` has `Hash#compact!` been added since Ruby 2.4 ... https://bugs.ruby-lang.org/issues/11818
- There is `Set` in collection libraries other than `Array` and `Hash`. But `Set` doesn't have `compact` and `compact!` methods.

It behaves the same as `compact` and `compact!` methods of `Array` and `Hash` as follows.

`Set#compact!`:

```ruby
# Removes all nil elements from self. Returns self if any elements removed, otherwise nil.
set = Set[1, 'c', nil]            #=> #<Set: {1, "c", nil}>
set.compact!                      #=> #<Set: {1, "c"}>
set                               #=> #<Set: {1, "c"}>

set = Set[1, 'c']                 #=> #<Set: {1, "c"}>
set.compact!                      #=> nil
set                               #=> #<Set: {1, "c"}>
```

`Set#compact`:

```ruby
# Returns a new Set containing all non-nil elements from self.
set = Set[1, 'c', nil]            #=> #<Set: {1, "c", nil}>
set.compact                       #=> #<Set: {1, "c"}>
set                               #=> #<Set: {1, "c", nil}>

set = Set[1, 'c']                 #=> #<Set: {1, "c"}>
set.compact                       #=> #<Set: {1, "c"}>
set                               #=> #<Set: {1, "c"}>
```

Pull Request ... https://github.com/ruby/ruby/pull/3617



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