Add nucleotide-count exercise

[keiravillekode]

We should define a datatype for the 4 nucleotides. Students should be asked to define Show, Eq and Ord.

This exercise will use Data.SortedMap

[keiravillekode]

This exercise has already been implemented, using a Record and without requiring the student to implement Show, Eq or Ord.

Keeping this open, to make sure we use Ord and Data.SortedMap in another exercise, and at least one exercise will ask students to define Show and Eq.