[ruby-core:75172] [Ruby trunk Feature#12306] Implement String #blank? #present? and improve #strip and family to handle unicode

Issue #12306 has been updated by Martin D端rst.

Assignee set to Yukihiro Matsumoto

Several comments, all in one post:

1) .blank? definitely cannot check for all problems in an input field, but it seems to be used very often because it very easily catches a frequent user mistake. More precise checking is more difficult, more application-dependent, and will catch less mistakes. So I'm not surprised that may programmers use it, even if it's just only a first step.

2) If we want to make Ruby 3 times faster by Ruby 3.0 (see Matz's keynote at last year's Ruby Kaigi), then ignoring a request to speed up functionality that may take up to 2% to 5% of time in the most widely used application of Ruby seems to be rather counter-purpose.

3) Regarding Unicode awareness, there are many ways to extend the definition of white space. See e.g. https://discourse.wicg.io/t/whitespace-is-hard-and-buggy-can-we-normalize-it/1436. The experience of Rails may be very valuable, but we'll have to look at it in detail.

4) Regarding backwards compatibility, for upcase/downcase/..., Matz has said that he's willing to make it backwards-incompatible in edge cases (if you have non-ASCII data, but *really* only want ASCII to change case). That might apply here, too. But we have to discuss it.

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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-58306

* Author: Sam Saffron
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
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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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