[#1] Convert Spy::Stats to use data+refresh pattern.

Instead of defining each statistic as a method on `Spy::Stats`, use
an internal hash, updated by calling `#refresh`. This makes `Spy::Stats`
mirror the approach of the other Spies.

Note that other Spies use internal arrays of hashes, whereas `Spy::Stats`
uses only a depth-1 internal hash. This doesn't affect usage, however,
and is compatible with enumerable `Spy::Dataspyable`.