--warn about unused variables

[ghost]

case model.view of
  All hasPaid ->
    -- hasPaid then not used

elm make would warn that hasPaid is unused.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Tracking in #98