Improve performance of implicit type conversion

[Watson1978]

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.
(

ruby/object.c

Lines 2634 to 2643 in 4f2db15

	if (strncmp(prefix, method, sizeof(prefix)-1) == 0) {
	const char *const meth = &method[sizeof(prefix)-1];
	for (i=0; i < numberof(conv_method_names); i++) {
	if (conv_method_names[i].method[0] == meth[0] &&
	strcmp(conv_method_names[i].method, meth) == 0) {
	m = conv_method_names[i].id;
	break;
	}
	}
	}

)

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

https://bugs.ruby-lang.org/issues/13341

[nurse]

LGTM but rb_convert_type2() is bad name; use rb_convert_type_with_id or something.
(I know you follow CRuby's old style, but we now consider it is bad style)

[Watson1978]

Rebased to HEAD (cc50ed4) in trunk branch