[Fixes #22] AutoConstToSet builds Set variants of constants automatically

[marcandre]

Here's another approach to deal with the fact that calling some_array.include? is a waste of CPU cycles.

If a module extends AutoConstToSet, then SOME_CONSTANT_SET will be a set created automatically from SOME_CONSTANT.

class Foo
 extend AutoConstToSet
 WORDS = %w[hello world].freeze
end
Foo::WORDS_SET # => Set['hello', 'world'].freeze

This has the advantage that both the Array and the Set versions are available, without too much verbosity.

It requires the rubyist to call SOME_CONSTANT_SET.include? instead of SOME_CONSTANT.include? but we could even have an internal cop for this.

[marcandre]

@rubocop-hq/rubocop-core how does this solution look?

[bbatsov]

My only concern is that this behavior is not particularly obvious and probably people will be a bit confused by it. A cop might help, though.

On the bright side - it is the least invasive option we've discussed so far, although I personally prefer a more explicit approach even if we have to write more code.

[marcandre]

If I make the ..._SET constants private, then it should be possible to revert this or go to any other solution without any impact...