Templates for practice exercises

Since we already have written templates for a good number of exercises, in this issue, I'll attempt to collect all the parameters that currently influence how an exercise is written. This will serve as a reference for:

* How tests are generally written
* How to handle error cases and input validation
* How to write stubs
* How to address inconsistencies in terminology between the problem specifications and the Clojure implementation
* How to decide on the types of input and output

The older issues that currently discuss these topics are:

* [How to handle input validation and other exceptions in exercises?](https://github.com/exercism/clojure/issues/670)
* [Descriptions and tests may be misleading regarding the type of both input and expected values](https://github.com/exercism/clojure/issues/671)
* [Template for tests](https://github.com/exercism/clojure/issues/673)
* [[v3] Template for Exercises](https://github.com/exercism/clojure/issues/305)
* [Add section to exercise READMEs for input/output format?](https://github.com/exercism/clojure/issues/229)

As part of the housecleaning, I'll be closing all these issues. We can reference them from this issue if we ever need them.