Cleaning up build file
Exporting data constructors from `Interpreter`
Providing a "printable" version of `PathingMap` to avoid `Show` conflict

As I write this, I'm on a plane, so I can't really look up a better way
to do this, but surely I should fix this situation later.  I think it
would be dumb if I couldn't more-seamlessly override the `Show` for 2D
arrays whenever I have a declared `PathingMap`.
Exporting things explicitly from `PathingMap`
For ease of use, providing typeclass instances for a few things
Making empty terrain to be blank
Improving clarity
Fixing `Interpreter` to flip the input's axes correctly

This is something that I've always found tricky, and I assumed that I
messed it up when I wrote the first implementation here in Haskell.

In the Scala version, the transformation is done by reassigning the
contents of arrays, but Haskell doesn't really allow that (and I'm
fine with that).

Solving it--like most things in Haskell--mostly just involved sitting
down and thinking about what the problem was that I wanted to solve.
Adding some tests for `Interpreter`
Deriving some free typeclasses
Being sure to export `PathingMap.getTerrain`
Fixing `Show` instance for `PathingGrid`s
Printing `PathingGrid`s with pretty borders
Adding tests for `PathingMap`