nucleotide-count: Use data type to represent nucleotides

[sshine]

Preserve the Data.Map.

References #694

[petertseng]

I see, so not changing the entire input, just the outputs.

You need to use a word from https://help.github.com/articles/closing-issues-using-keywords/, not "References". Please do that.

[petertseng]

References #964

doesn't exist. it's 694. (so please fix to use correct issue number)

[sshine]

@petertseng With a few tries, I've addressed both issues.

And yes, I figured accepting broken strings was in the spirit of the assignment, like broken DNA. :-)

[petertseng]

fantastic! let's get this merged (as soon as tests pass)!

I just remembered that this wants a version increase (it's documented-as-undocumented in #538), so I'll add it. Wouldn't want to hold this up over that.

[petertseng]

Very good, thank you!

[sshine]

Three months later, giving my first review writing mentor notes to this exercise and realizing my own version doesn't have the data Nucleotide definition, I ask myself: "Who the hell added this in the meantime?"