Have Iterator implement Enumerable

[AndrewSisley]

Remove the Valid function. Have Seek, Next, and maybe store.Iter return booleans. Add a Reset function.

Strongly consider not directly embedding Enumerable within the Iterator interface, but instead heavily document and test that it implements it - this will save a production code reference to the Enumerable package.

This change will facilitate #23 but do not go out of your way to include that within this work-item.