[ruby-dev:50024] [Ruby trunk Bug#13341] Improve performance of implicit type conversion

Issue #13341 has been reported by watson1978 (Shizuo Fujita).

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Bug #13341: Improve performance of implicit type conversion
https://bugs.ruby-lang.org/issues/13341

* Author: watson1978 (Shizuo Fujita)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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At least, Array#flatten will be faster around 20%.
Seems that strncmp() & strcmp() in convert_type() are slightly heavy to look up the method's id for type conversion.
(https://github.com/ruby/ruby/blob/4f2db15b42d7b8eb5b304a92ba2296632dba3edf/object.c#L2634-L2643)

This patch will use known method's id directly.

### Before
~~~
                                             user     system      total        real
Array#flatten (rb_check_convert_type2)   1.000000   0.000000   1.000000 (  1.001917)
Array#+ (rb_convert_type2)               1.010000   0.000000   1.010000 (  1.006383)
~~~

### After
~~~
                                             user     system      total        real
Array#flatten (rb_check_convert_type2)   0.830000   0.000000   0.830000 (  0.833411)
Array#+ (rb_convert_type2)               0.950000   0.000000   0.950000 (  0.953832)
~~~

### Test Code
~~~
require 'benchmark'

Benchmark.bmbm do |x|

  ary = []
  100.times { |i| ary << i }
  array = [ary]

  x.report "Array#flatten (rb_check_convert_type2)"do
    100000.times do
      array.flatten
    end
  end

  x.report "Array#+ (rb_convert_type2)"do
    class Foo
      def to_ary
        [1,2,3]
      end
    end
    obj = Foo.new

    2000000.times do
      array + obj
    end
  end

end
~~~

### Patch
The patch is in https://github.com/ruby/ruby/pull/1537



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