[elixir] Remove unnecessary Enum.map from HashDict.new

It looks like you can just initialize a HashDict with some list and then
a function to map values, i.e., no need for an explicit Enum.map.
[elixir] Use the DNA module constant

I had passed over this earlier, but there's no need to actually declare
my list of nucleotides since a constant is already wired up via a
[module attribute][ma].

[ma]: http://elixir-lang.org/getting_started/14.html#toc_2
[elixir] Tidy DNA anonymous fn syntax w/ partials

Elixir's [partial application of functions][1] can help clean things up
to be more concise.

[1]: http://elixir-lang.org/crash-course.html#toc_22