funcr doesn't handle recursive pointers well

[thockin]

type T struct {    
    P *T    
}    

func example(log logr.Logger) {    
    t := T{}    
    t.P = &t    
    log.Info("recurse", "t", t)     
}

yields a (predictable) stack overflow.

We probably should add a max-depth option or track a stack of pointers across pretty calls and not print pointers already in the stack. Or both.