Improve performance of embedded string allocation

[peterzhu2118]

Non-VWA embedded string allocation had a performance regression. This commit improves performance of non-VWA embedded string allocation.

Benchmark script:

benchmark:
  ary: |
      a = []; b = []; c = []; d = []; e = []
      a = []; b = []; c = []; d = []; e = []
      a = []; b = []; c = []; d = []; e = []
      a = []; b = []; c = []; d = []; e = []
      a = []; b = []; c = []; d = []; e = []
      a = []; b = []; c = []; d = []; e = []
  str: |
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"
      a = "str"; b = "str"; c = "str"; d = "str"; e = "str"

Before this commit:

Warming up --------------------------------------
                 ary                                  1.919M i/s -      2.013M times in 1.049053s (521.12ns/i)
                 str                                  1.731M i/s -      1.748M times in 1.010009s (577.66ns/i)
Calculating -------------------------------------
                     /home/peter/.rubies/ruby-3.0.3/bin/ruby  /home/peter/src/ruby/install/bin/ruby
                 ary                                  1.962M                                 1.870M i/s -      5.757M times in 2.934596s 3.077806s
                 str                                  1.784M                                 1.122M i/s -      5.193M times in 2.911832s 4.630396s

Comparison:
                              ary
/home/peter/.rubies/ruby-3.0.3/bin/ruby:   1961712.7 i/s
/home/peter/src/ruby/install/bin/ruby:   1870434.4 i/s - 1.05x  slower

                              str
/home/peter/.rubies/ruby-3.0.3/bin/ruby:   1783550.4 i/s
/home/peter/src/ruby/install/bin/ruby:   1121588.6 i/s - 1.59x  slower

After this commit:

Warming up --------------------------------------
                 ary                                  1.899M i/s -      2.000M times in 1.052945s (526.47ns/i)
                 str                                  1.741M i/s -      1.756M times in 1.008239s (574.22ns/i)
Calculating -------------------------------------
                     /home/peter/.rubies/ruby-3.0.3/bin/ruby  /home/peter/src/ruby/install/bin/ruby
                 ary                                  1.963M                                 1.885M i/s -      5.698M times in 2.902366s 3.022505s
                 str                                  1.754M                                 1.649M i/s -      5.224M times in 2.979164s 3.169046s

Comparison:
                              ary
/home/peter/.rubies/ruby-3.0.3/bin/ruby:   1963341.0 i/s
/home/peter/src/ruby/install/bin/ruby:   1885301.6 i/s - 1.04x  slower

                              str
/home/peter/.rubies/ruby-3.0.3/bin/ruby:   1753669.0 i/s
/home/peter/src/ruby/install/bin/ruby:   1648592.8 i/s - 1.06x  slower