Implemented doubleEveryOther function.
Grouped imports as per https://github.com/tibbe/haskell-style-guide/blob/49f7f9e2ad7786fca3afe147e2b6da382aa724a4/haskell-style.md#imports

As the document describes:

1. Standard library imports
2. Related third party imports
3. Local application/library specific imports.

A blank line separates each group of imports.
The imports in each group are be sorted alphabetically, by module name.
Removed extra whitespace from imports. This has been originally set that
way by the Haskeleton template. I'm not currently sure why there should be
so much extra whitespace, thus it's now removed.
Grouped property-based tests in homeworks (e.g. 1, 2, .. 10).