This class is a stripped-down version of [troposphere/resolver.py](https://github.com/fungusakafungus/troposphere/blob/0035222fb5f01220b429063fa2b76ed53088d1bf/troposphere/resolver.py) from
#30, where it indeed had a state (a dict of objects known to the resolver)

It doesn't make much sense for this bunch of functions to be a class now. I'll refactor it.