Optionally pass a function instead of "actual"

The function will be printed in its entirety in the stack trace.

I think this is a reasonable alternative to writing hyper-detailed assert messages.  Instead, one can submit an arbitrarily granular function, and then one gets the opportunity to inspect that function whenever there is an error.

I *think* that being presented with the source of a test function every time there is an error, is going to be pretty helpful...