feature: Reentrant-lock version #356
Description
I have a struct that represents an expression tree and holds references to itself in ArcIntern. Since ArcIntern uses DashMap internally, I noticed deadlocks while parsing and evaluating expressions. It would be nice if a DashMap based on reentrant locks were available; that way, deadlocks would only occur in the cases where they'd also occur in a HashMap<K,RwLock<V>>. parking_lot::RwLock offers the read_recursive method for this (although that should probably be an opt-in because apparently it can starve writers).