Make some kind of guarding for advancing

[AaronKutch]

There are some pretty horrendous sporadic bugs that can happen with the interaction between removal and insertion with chain advancing. It is easy to forget the caution on the advancer functions. We need some way to guard against this problem, perhaps by making advancer() take the arena by value and have a zero cost wrapper returned that forwards every thing except the affected functions, then the arena can be converted back to its normal type. Has the drawback of a lot of code churn and making the advancer loop setup less ideal. We also need to handle references to arenas.