Consider adding a concrete lifetime to the Visit trait

[KodrAus]

It looks like most of the work to support visiting borrowed data for some concrete lifetime (instead of just for any lifetime) is already done, the Visit trait just doesn't surface a lifetime that implementors can use to stash values across calls.

I at least find introducing lifetimes late in an API design can get a bit hairy so it might something to explore sooner rather than later 🙂

[KodrAus]

This may turn out to be less important for valuable once it has some pointer concept (#59) if you can logically borrow a piece of data by storing a borrow to the overall Valuable and a pointer to the specific field you want to borrow.