Syntax fixes for compatiblity with Crystal 1.7.1

[skinnyjames]

Hi @ziprandom! Thank you for this library. I'd like to bring it up to date as to bring your cltk project up to date.

This commit brings this lib up to date with newer Crystal by addressing explicit return types and spec behavior.

• Uses describe instead nesting it blocks
• Uses explicit return types for abstract def implementations
• Uses a splatted Tuple generic

I'll work off my branches for now, but I'd love to see your great work here come up to date!