Create stub files for all exercises about crystal

[mhelmetag]

I believe we've had stubs since the beginning. For crystal, they're blank files in the src directory of the exercise that work with the crystal spec command and have a comment to add your solution there.

I think we're all good then.

[nathanchere]

I don't think it's all good. Talking about pain points for getting started, most other language tracks do this a lot better. For example:

module Gigasecond
  def from(start : Time) : Time
    # Please implement your solution to gigasecond in this file
  end
end

is a lot better than

# Please implement your solution to gigasecond in this file

I think it would make a significant difference to making the Crystal track more welcoming and encouraging for newcomers.

[mhelmetag]

So I have seen tracks do it both ways (having a stub with a method, module, class, etc) or an empty file and there are pros and cons of both.

I think that having a default stub with a module (or anything for that matter) is too prescriptive. I think the blank file allows the solver to try anything that fits. I also think it's a nice intro to TDD and "simple green". Crystal can't find the namespace Gigasecond so make a class or module or whatever. Crystal can't find a method in that namespace so make that method. Etc.

I think we could add stubs in the first few "core" exercises but... we first need to pick which exercises those are.

If you have some suggestions for "core" exercises (or any additions to round out that core set), it'd be awesome to have a PR with better stubs for those exercises and the updated track config file.