Add version_restriction_test

[straight-shoota]

Also transform helper methods to class methods for better isolation and testability.

This is a preliminary step for implementing #184

[ysbaddaden]

Adding tests is great. Breaking the includable modules for class methods is an opinionated stylistic change. Without further commits to support this change, it's hard to understand and validate it.

If there is a bug in NaturalSort, a fix would be very welcome.

Last but not least, the unit test file should symmetric to the file being tested. For example test/helpers/versions_test.cr

[straight-shoota]

It may be stylistic, but it's foremost practical as it makes these methods easier to test. I don't see why breaking the includes is a bad thing, if anything it clarifies the API.

There is a bug in resolving ~> restrictions. I won't fix NaturalSort but propose an improved algorithm instead (which doesn't have this bug).

From my perspective this PR is a simple improvement, self-contained and easy to review. It stands by itself and shouldn't depend on any planned changes.

Moved the file.

[ysbaddaden]

Thanks for the tests. They were applied in #249.