Demonstrate that we raise an error for spec funs of wrong arity.

If a spec function doesn't have arity 1 this is a problem, as mentioned by Mike
in #8. `normalize_spec/2` was written to exclude such bad arity functions but
this was never explicitly asserted.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #10 from blt/fail_on_bad_arity

Demonstrate that we raise an error for spec funs of wrong arity.