Introduce Temporary.using for ensuring directory cleanup after use

Raised from Ivory [PR comment](https://github.com/ambiata/ivory/commit/79699cfcf8c80b77000865c8108036425249684c#commitcomment-6873287)
Happy to be told that `AfterExample` is a cleaner/easier approach. Wondering whether we should just create a specs trait instead. Thoughts?