This refactor removes reliance on Prelude in favour of using it only where appropriate and re-implementing it otherwise.

Prelude is appropriate for `instance Show` (and therefore, String), since GHCi relies heavily on this type-class.

There is now only one top-level module (`Course`), since the others were poorly categorised. A reasonable categorisation is desirable, but needs more work to determine.

Some exercises have been removed.

Version number is bumped to indicate no compatiblity.