[ruby-core:84008] [Ruby trunk Feature#14146] Improve performance of creating Hash object

Issue #14146 has been reported by watson1978 (Shizuo Fujita).

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Feature #14146: Improve performance of creating Hash object
https://bugs.ruby-lang.org/issues/14146

* Author: watson1978 (Shizuo Fujita)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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When generate Hash object, the heap area of st_table will be always allocated in internally
and seems it take a time.

To improve performance of creating Hash object,
this patch will reduce count of allocating heap areas for st_table by reuse them.

	Performance of creating Hash literal -> 1.53 times faster.

### Before
```
$ ./miniruby -v -I. -I../benchmark-ips/lib ~/tmp/bench/literal.rb
ruby 2.5.0dev (2017-11-28 hash 60926) [x86_64-darwin17]
Warming up --------------------------------------
        Hash literal    51.544k i/100ms
Calculating -------------------------------------
        Hash literal    869.132k (賊 1.1%) i/s -      4.381M in   5.041574s
```

### After
```
$ ./miniruby -v -I. -I../benchmark-ips/lib ~/tmp/bench/literal.rb
ruby 2.5.0dev (2017-11-28 hash 60926) [x86_64-darwin17]
Warming up --------------------------------------
        Hash literal    63.068k i/100ms
Calculating -------------------------------------
        Hash literal      1.328M (賊 2.3%) i/s -      6.685M in   5.037861s
```

### Test code
```
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report "Hash literal" do |loop|
    count = 0
    while count < loop
      hash = {foo: 12, bar: 34, baz: 56}

      count += 1
    end
  end
end
```

### Patch
https://github.com/ruby/ruby/pull/1766




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