Improvement: Add test cases for graph hashing & isomorphism
Improvement: Remove print statements

Yeah those should never have been comitted.
Improvement: Use clojure.core.memoize

Pull in the clojure.core.memoize library in the interests of efficient
memory usage. This may come at a performance hit, but will ensure that
structural-hash can't suck up and sit on huge amounts of memory until
the JVM dies.
Improvement: Clean up my whitespace mess