Utilize BurntSushi's toml-test to test

@BurntSushi created [toml-test](https://github.com/BurntSushi/toml-test)
as a language-agnostic means of testing TOML parsers. it's more well-maintained
than our test suite so let's use it.

Currently, it hangs with the interface I yoinked from @cespare in
https://gist.github.com/cespare/5052442. Know why?