Optimization of REXML::Document#add, #add_element

[naitoh]

Benchmark

$ benchmark-driver benchmark/parse_comment.yaml
                         before       after  before(YJIT)  after(YJIT)
           top_level    48.497k     54.645k        7.951k       8.288k i/s -     100.000 times in 0.002062s 0.001830s 0.012577s 0.012066s
          in_doctype    48.615k     50.994k        7.930k       7.820k i/s -     100.000 times in 0.002057s 0.001961s 0.012611s 0.012787s
       after_doctype    63.251k     64.061k        9.569k       9.434k i/s -     100.000 times in 0.001581s 0.001561s 0.010450s 0.010600s
       many_comments     67.653     611.015       117.191      929.290 i/s -     100.000 times in 1.478125s 0.163662s 0.853309s 0.107609s

Comparison:
                        top_level
               after:     54644.8 i/s
              before:     48496.6 i/s - 1.13x  slower
         after(YJIT):      8287.7 i/s - 6.59x  slower
        before(YJIT):      7951.0 i/s - 6.87x  slower

                       in_doctype
               after:     50994.4 i/s
              before:     48614.5 i/s - 1.05x  slower
        before(YJIT):      7929.6 i/s - 6.43x  slower
         after(YJIT):      7820.4 i/s - 6.52x  slower

                    after_doctype
               after:     64061.5 i/s
              before:     63251.1 i/s - 1.01x  slower
        before(YJIT):      9569.4 i/s - 6.69x  slower
         after(YJIT):      9434.0 i/s - 6.79x  slower

                    many_comments
         after(YJIT):       929.3 i/s
               after:       611.0 i/s - 1.52x  slower
        before(YJIT):       117.2 i/s - 7.93x  slower
              before:        67.7 i/s - 13.74x  slower

• YJIT=ON : 0.98 - 7.93x faster
• YJIT=OFF : 1.01 - 9.02x faster