Method to check validity of CID

[vbaranov]

It would be useful for lib to provide method Cid.cid? to check validity of the given CID as here. Providing of this method will enable integration of excid package into Blockscout in order to fulfil this task.

Todo

• create the Cid.cid?/1 function to check validity of a cid 🧑‍💻
• publish new version of excid package 📦

[nelsonic]

@vbaranov great idea! 💡 Let's add this for sure! ➕