Add new Performance/ReverseFirst cop

[fatkodima]

There is no need to reverse the whole array to get last n items in order from end to begin.

# bad
array.reverse.first(5)
array.reverse.first

# good
array.last(5).reverse
array.last

Benchmark

require 'bundler/inline'

gemfile(true) do
  gem 'benchmark-ips'
end

ARRAY = (1..100).to_a

Benchmark.ips do |x|
  x.report('Array.last(n).reverse')  { ARRAY.last(5).reverse }
  x.report('Array.reverse.first(n)') { ARRAY.reverse.first(5) }
  x.compare!
end

Results

Warming up --------------------------------------
Array.last(n).reverse
                       515.468k i/100ms
Array.reverse.first(n)
                       101.975k i/100ms
Calculating -------------------------------------
Array.last(n).reverse
                          4.544M (± 8.7%) i/s -     22.681M in   5.034133s
Array.reverse.first(n)
                        913.988k (± 9.9%) i/s -      4.589M in   5.080680s

Comparison:
Array.last(n).reverse:  4543639.0 i/s
Array.reverse.first(n):   913988.0 i/s - 4.97x  (± 0.00) slower

Obviously, this scales linearly with array size.

[fatkodima]

Updated with missing tests.

[koic]

Thanks!