[ruby-core:86093] [Ruby trunk Bug#14599] Improve Pathname performance

Issue #14599 has been reported by watson1978 (Shizuo Fujita).

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Bug #14599: Improve Pathname performance
https://bugs.ruby-lang.org/issues/14599

* Author: watson1978 (Shizuo Fujita)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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If it will not use special variables (like $1, $&, $`...),
it can improve the performance by using Regexp#match? instead of Regexp#=~.
Because Regexp#=~ will generate the objects to special variables by pattern matching.

This patch will replace Regexp#=~ without special variables to Regexp#match?.
(Excludes https://github.com/ruby/ruby/blob/trunk/ext/pathname/lib/pathname.rb#L144-L153)

## Environment
* OS : Ubuntu 17.10
* Compiler : gcc version 7.2.0
* CPU : Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
* Memory : 16 GB

## TL;DR
                            | before | after  | Speed up
--------------------------- | ------ | ------ | --------
Pathname#absolute?          | 142836 | 198487 | 39.0%
Pathname#cleanpath          |  60706 |  79415 | 30.8%
Pathname#root?              | 603806 | 759157 | 25.7%
Pathname#absolute?          | 142592 | 197859 | 38.8%
Pathname#each_filename      | 115600 | 152982 | 32.3%
Pathname#ascend             |  50494 |  63606 | 26.0%
Pathname#+                  | 100550 | 130372 | 29.7%
Pathname#join               |  46673 |  60994 | 30.7%
Pathname#relative_path_from |  28362 |  37494 | 32.2%

## Before
```
Calculating -------------------------------------
  Pathname#absolute?    142.836k (± 0.1%) i/s -    722.304k in   5.056884s
  Pathname#cleanpath     60.706k (± 0.1%) i/s -    306.764k in   5.053305s
      Pathname#root?    603.806k (± 0.3%) i/s -      3.062M in   5.071696s
  Pathname#absolute?    142.592k (± 0.1%) i/s -    720.846k in   5.055301s
Pathname#each_filename
                        115.600k (± 0.1%) i/s -    586.818k in   5.076292s
     Pathname#ascend     50.494k (± 0.1%) i/s -    255.301k in   5.056049s
          Pathname#+    100.550k (± 0.1%) i/s -    509.630k in   5.068433s
       Pathname#join     46.673k (± 0.1%) i/s -    236.433k in   5.065696s
Pathname#relative_path_from
                         28.362k (± 0.0%) i/s -    143.728k in   5.067640s
```

## After
```
Calculating -------------------------------------
  Pathname#absolute?    198.487k (± 0.1%) i/s -    995.665k in   5.016272s
  Pathname#cleanpath     79.415k (± 0.1%) i/s -    404.406k in   5.092344s
      Pathname#root?    759.157k (± 0.0%) i/s -      3.800M in   5.005072s
  Pathname#absolute?    197.859k (± 0.1%) i/s -    995.720k in   5.032494s
Pathname#each_filename
                        152.982k (± 0.1%) i/s -    775.555k in   5.069607s
     Pathname#ascend     63.606k (± 0.0%) i/s -    320.862k in   5.044560s
          Pathname#+    130.372k (± 0.1%) i/s -    660.856k in   5.068991s
       Pathname#join     60.994k (± 0.1%) i/s -    305.068k in   5.001626s
Pathname#relative_path_from
                         37.494k (± 0.4%) i/s -    189.124k in   5.044146s
```

## Benchmark code
```ruby
require 'pathname'
require 'benchmark/ips'

Benchmark.ips do |x|
  root  = Pathname.new('/')
  path1 = Pathname.new('/path/to/some/file1.rb')
  path2 = Pathname.new('/path/to/some/file2.rb')

  x.report("Pathname#absolute?") do
    path1.absolute?
  end

  x.report("Pathname#cleanpath") do
    Pathname.new('/path/to/some/file.rb').cleanpath
  end

  x.report("Pathname#root?") do
    path1.root?
  end

  x.report("Pathname#absolute?") do
    path1.absolute?
  end

  x.report("Pathname#each_filename") do
    path1.each_filename { |file| }
  end

  x.report("Pathname#ascend") do
    path1.ascend { |path| }
  end

  x.report("Pathname#+") do
    path1 + path2
  end

  x.report("Pathname#join") do
    path1.join("../file3.rb")
  end

  x.report("Pathname#relative_path_from") do
    path1.relative_path_from(root)
  end
end
```

## Patch
https://github.com/ruby/ruby/pull/1836



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