Commits on May 6, 2020

1. Add new Performance/DeletePrefix and Performance/DeleteSuffix cops …

   This PR adds new `Performance/DeletePrefix` and `Performance/DeleteSuffix` cops.
   
   In Ruby 2.5, `String#delete_prefix` and `String#delete_suffix` have been added.
   
   - https://ruby-doc.org/core-2.5.8/String.html#method-i-delete_prefix
   - https://ruby-doc.org/core-2.5.8/String.html#method-i-delete_suffix
   
   `Performance/DeletePrefix` cop identifies places where `gsub(/\Aprefix/, '')`
   can be replaced by `delete_prefix('prefix')`.
   
   ```ruby
   # bad
   str.gsub(/\Aprefix/, '')
   str.gsub!(/\Aprefix/, '')
   str.gsub(/^prefix/, '')
   str.gsub!(/^prefix/, '')
   
   # good
   str.delete_prefix('prefix')
   str.delete_prefix!('prefix')
   ```
   
   The `delete_prefix('prefix')` method is faster than `gsub(/\Aprefix/, '')`.
   
   ```console
   % ruby -v
   ruby 2.5.8p224 (2020-03-31 revision 67882) [x86_64-darwin17]
   
   % cat bench.rb
   require 'benchmark/ips'
   
   Benchmark.ips do |x|
     str = 'foobar'
   
     x.report('gsub')           { str.gsub(/bar\z/, '') }
     x.report('gsub!')          { str.gsub(/bar\z/, '') }
     x.report('delete_suffix')  { str.delete_suffix('bar') }
     x.report('delete_suffix!') { str.delete_suffix('bar') }
     x.compare!
   end
   
   % ruby bench.rb
   Warming up --------------------------------------
                   gsub    46.814k i/100ms
                  gsub!    46.896k i/100ms
          delete_suffix   211.337k i/100ms
         delete_suffix!   208.332k i/100ms
   Calculating -------------------------------------
                   gsub    546.500k (± 1.3%) i/s -      2.762M in 5.054918s
                  gsub!    551.054k (± 1.2%) i/s -      2.767M in 5.021747s
          delete_suffix      4.780M (± 1.1%) i/s -     24.092M in 5.040850s
         delete_suffix!      4.770M (± 1.1%) i/s -     23.958M in 5.022823s
   
   Comparison:
          delete_suffix:  4780060.8 i/s
         delete_suffix!:  4770419.3 i/s - same-ish: difference falls within error
                  gsub!:   551054.2 i/s - 8.67x  slower
                   gsub:   546500.1 i/s - 8.75x  slower
   ```
   
   `Performance/DeleteSuffix` cop is the same.

   

   koic committed May 6, 2020
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