Add Lucky.root redirection to Dir.current through exception.

[stephendolan]

Purpose

Adds Lucky.root method which, when used in an application will raise a compile-time error redirecting users to leverage Dir.current. Resolves #1198.

Description

Context can be found in #1198, including around the decision to make this a compile-time error.

No specs added since it's raised at compile-time, and not post-build.

Not 100% sure about how this will look in code documentation with just the single comment, please let me know if I need to improve that.

Didn't add a CHANGELOG entry since I didn't see an existing "UNRELEASED" section, but happy to help add that in.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[paulcsmith]

Suggestion to add a bit more background info in the docs. Other than that this looks great!

[jwoertink]

This looks good to me 👍

Also, don't worry about the CHANGELOG entry. I think we should just remove that mention. I'd rather just handle adding in all of the entries at once than having each PR go adding their own in.

[stephendolan]

Cool, sounds good. @jwoertink / @paulcsmith I'll make a note in #1205 to discuss removing the CHANGELOG entry requirement.

[paulcsmith]

This looks great. Thanks @stephendolan!