Check slow hash accessing in Array#sort by Performance/CompareWithBlock cop

[pocke]

Currently, Performance/CompareWithBlock cop accepts the following code.

array.sort { |a, b| a[:foo] <=> b[:foo] }

But the code has same issue as a.foo <=> b.foo.
So, by this change, the cop registers an offense for this code.

Benchmark

require 'benchmark'

array10000 = 10000.times.map do |i|
  {value: i}
end.shuffle

array100 = 100.times.map do |i|
  {value: i}
end.shuffle

array10 = 10.times.map do |i|
  {value: i}
end.shuffle

Benchmark.bm(15) do |x|
  x.report('sort_by 10000'){500.times{array10000.sort_by{|a| a[:value]}}}
  x.report('sort    10000'){500.times{array10000.sort{|a, b| a[:value] <=> b[:value]}}}

  x.report('sort_by 100'){50000.times{array100.sort_by{|a| a[:value]}}}
  x.report('sort    100'){50000.times{array100.sort{|a, b| a[:value] <=> b[:value]}}}

  x.report('sort_by 10'){500000.times{array10.sort_by{|a| a[:value]}}}
  x.report('sort    10'){500000.times{array10.sort{|a, b| a[:value] <=> b[:value]}}}
end

$ ruby test.rb
                      user     system      total        real
sort_by 10000     2.470000   0.000000   2.470000 (  2.480009)
sort    10000     4.790000   0.010000   4.800000 (  4.797943)
sort_by 100       1.140000   0.000000   1.140000 (  1.146861)
sort    100       1.910000   0.000000   1.910000 (  1.910717)
sort_by 10        0.880000   0.000000   0.880000 (  0.871086)
sort    10        1.000000   0.000000   1.000000 (  1.002856)

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

emthod -> method

[pocke]

@bbatsov Thanks. I fixed and rebaesd.