Small improvement to Style/MapIntoArray's autocorrection detection.

This cop can only autocorrect in a void context, and currently it only checks (kw)begin/(num)block contexts. This PR adapts some of the new logic from Lint/Void to expand void context detection to ensure, def, defs, and for contexts by delegating the responsibility to rubocop-ast.

e.g.

def initialize
  dest = []
  list.each { dest << _1 } # 🆕 can now be autocorrected since it's in constructor
end

def self.name=(value)
  dest = []
  list.each { dest << _1 } # 🆕 can now be autocorrected since it's in an assignment method
end

def foo
  bar
ensure
  dest = []
  list.each { dest << _1 } # 🆕 can now be autocorrected since ensure is always a void context
end

for i in list
  dest = []
  list.each { dest << _1 } # 🆕 can now be autocorrected since it's in a for loop
end

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.