Check for unused function inputs

[rigelrozanski]

When function inputs are no longer used, they should either be removed from the function - if they can't be removed they need to referenced in the function - this should be built into CI.

REF: #994 (comment)

CC: @melekes - ever seen anything that can do this?

[rigelrozanski]

https://github.com/mvdan/unparam looks good - kind of confusing to use however -> try:
unparam ./... or unparam ./x/... for instance