## Why?
Set.new() instances of the prefixes variable can be reused, reducing initialization costs.

## Result
```
RUBYLIB= BUNDLER_ORIG_RUBYLIB= /Users/naitoh/.rbenv/versions/3.3.3/bin/ruby -v -S benchmark-driver /Users/naitoh/ghq/github.com/naitoh/rexml/benchmark/parse.yaml
ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin22]
Calculating -------------------------------------
                         before       after  before(YJIT)  after(YJIT)
                 dom     17.714      17.658        32.898       33.247 i/s -     100.000 times in 5.645176s 5.663160s 3.039707s 3.007755s
                 sax     25.280      25.281        47.483       49.990 i/s -     100.000 times in 3.955694s 3.955534s 2.106006s 2.000389s
                pull     29.048      29.061        59.944       61.498 i/s -     100.000 times in 3.442599s 3.441014s 1.668222s 1.626060s
              stream     28.181      28.440        52.340       55.078 i/s -     100.000 times in 3.548546s 3.516169s 1.910599s 1.815599s

Comparison:
                              dom
         after(YJIT):        33.2 i/s
        before(YJIT):        32.9 i/s - 1.01x  slower
              before:        17.7 i/s - 1.88x  slower
               after:        17.7 i/s - 1.88x  slower

                              sax
         after(YJIT):        50.0 i/s
        before(YJIT):        47.5 i/s - 1.05x  slower
               after:        25.3 i/s - 1.98x  slower
              before:        25.3 i/s - 1.98x  slower

                             pull
         after(YJIT):        61.5 i/s
        before(YJIT):        59.9 i/s - 1.03x  slower
               after:        29.1 i/s - 2.12x  slower
              before:        29.0 i/s - 2.12x  slower

                           stream
         after(YJIT):        55.1 i/s
        before(YJIT):        52.3 i/s - 1.05x  slower
               after:        28.4 i/s - 1.94x  slower
              before:        28.2 i/s - 1.95x  slower

```

YJIT=ON : 1.01x - 1.05x faster
YJIT=OFF : 0.99x - 1.00x faster