Add Lint/UslessArraySplat

[owst]

This new cop will detect useless splats of array literals:

# bad:
x, y = *[1, 2, 3]
# good:
x, y = [1, 2, 3]

# bad: 
x, y = *%w(a b c)
# good:
x. y = %w(a b c)

Also means that #3069 is no longer an issue, since the splat is removed before freeze is added.

Before submitting a PR make sure the following are checked:

• Wrote good commit messages.
• 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.

[bbatsov]

Do people really write code like this? :-)

You've got a typo in your commit message.

[owst]

Good spot on the typo! Re: writing this sort of code, I wouldn't :-) but I got the idea from the report of #3069. AFAICT, it's not possible to correct

FOO = *[1, 2, 3]

so that Style/MutableConstant doesn't trigger:

FOO = *[1, 2, 3].freeze

isn't right, and trying

(*[1, 2, 3]).freeze

is a syntax error (hence rubocop gets into a loop trying to fix the issue). However, with this new cop, the * is first removed, so that the code becomes

FOO = [1, 2, 3].freeze

[bbatsov]

Makes sense. Thanks!