[ruby-core:87384] [Ruby trunk Feature#12306] Implement String #blank? #present? and improve #strip and family to handle unicode

Issue #12306 has been updated by nobu (Nobuyoshi Nakada).


As `String#upcase` and family are Unicode case aware now, so the second feels somewhat reasonable.


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Feature #12306: Implement String #blank? #present? and improve #strip and family to handle unicode
https://bugs.ruby-lang.org/issues/12306#change-72370

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
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Time and again there have been rejected feature requests to Ruby core to implement `blank` and `present` protocols across all objects as ActiveSupport does. I am fine with this call and think it is fair. 

However, for the narrow case of String having `#blank?` and `#present?` makes sense. 

- Provides a natural extension over `#strip`, `#lstrip` and `#rstrip`. `("   ".strip.length == 0) == "    ".blank?`

- Plays nicely with ActiveSupport, providing an efficient implementation in Ruby core: see: https://github.com/SamSaffron/fast_blank, implementing blank efficiently requires a c extension. 

However, if this work is to be done, `#strip` and should probably start dealing with unicode blanks, eg: 

```
irb(main):008:0> [0x3000].pack("U")
=> "　"
irb(main):009:0> [0x3000].pack("U").strip.length
=> 1
```

So there are 2 questions / feature requests here

1. Can we add blank? and present? to String? 
2. Can we amend strip and family to account for unicode per: https://github.com/SamSaffron/fast_blank/blob/master/ext/fast_blank/fast_blank.c#L43-L74



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