Clear results when a function changes?

[mtorpey]

As @orlitzky points out, a function could be changed (for example, to fix a bug or change other behaviour) and yet the cache will persist, and by default it will be used simply based on the name of the function, loading cached results that are no longer valid.

At the moment we can get around this by specifying a custom funcname that uniquely identifies this version of the function, but perhaps something more resilient would be in order. Should we store a hash of the function's source code, or the name and version of the package in which it resides?

This overlaps with questions about namespaces. I'd welcome anyone's opinion, and will be asking people for their thoughts in the coming weeks.