Add strong count function to ArenaArc

[lenianiva]

This may be useful for hashconsing and bookkeeping

Example use case: I want to have a pool of hashconsigned AST objects, and I would store them in a hashmap HashMap<Expr, ArenaArc<Expr>>. Then if I want to garbage collect the consign, I can simply delete all (k, v) pairs such that v.strong_count() == 1.