Prefer String#start_with? over Regexp.match

[amatsuda]

Here's trivial performance improvement on IPv4 in_addr.

Benchmark result is as follows:

['0', '123', '03'].each do |s|
  p s
  Benchmark.ips do |x|
    x.report('old') { s.match(/\A0./) }
    x.report('new') { (s != '0') && s.start_with?('0') }
    x.compare!
  end
end


"0"
Warming up --------------------------------------
                 old   578.470k i/100ms
                 new     1.460M i/100ms
Calculating -------------------------------------
                 old      7.522M (± 0.7%) i/s -     38.179M in   5.076216s
                 new     24.614M (± 0.8%) i/s -    124.136M in   5.043628s

Comparison:
                 new: 24614178.4 i/s
                 old:  7521547.2 i/s - 3.27x  (± 0.00) slower

"123"
Warming up --------------------------------------
                 old   652.257k i/100ms
                 new     1.234M i/100ms
Calculating -------------------------------------
                 old      8.479M (± 0.6%) i/s -     42.397M in   5.000174s
                 new     18.665M (± 0.8%) i/s -     93.785M in   5.024798s

Comparison:
                 new: 18665473.8 i/s
                 old:  8479356.1 i/s - 2.20x  (± 0.00) slower

"03"
Warming up --------------------------------------
                 old   392.351k i/100ms
                 new     1.254M i/100ms
Calculating -------------------------------------
                 old      4.659M (± 1.3%) i/s -     23.541M in   5.053968s
                 new     19.023M (± 0.7%) i/s -     95.338M in   5.011941s

Comparison:
                 new: 19023166.8 i/s
                 old:  4658796.2 i/s - 4.08x  (± 0.00) slower

[mitchellhenke]

Thanks for implementing this, would be awesome to get this change merged!