Replace Crystal::Select::When with Crystal::When

[HertzDevil]

Alternative to / closes #14493

[straight-shoota]

This is probably a good idea, yet we should consider potential consequences. I don't think there's a huge impact. #14493 (comment) mentions smaller memory footprint, but that's not very relevant considering select statements are pretty rare and typically have only a handfull of branches.

There's a chance that the syntax for when in select and case might evolve into different directions in the future. I don't see anything like that happening and if it does we could still consider going back to different types.