Add a way to deterministically transition to CollectionPhase::Collecting

[kyren]

This is necessary when trying to use finalization, because you want to be completely out of the marking phase once you have decided finalization is finished for this cycle.

[kyren]

Changed MarkedArena::collect to MarkedArena::start_collecting

Added a drive by change from self.metrics.allocation_debt() <= target_debt to !(self.metrics.allocation_debt() > target_debt) out of NaN paranoia.

Added an EarlyStop enum for Context::do_collection, rather than relying on another custom method.

Changed MarkedArena to own a &mut Arena instead of &Arena. These are equivalent since every method that produces a MarkedArena takes &mut self and Rust can't downgrade mutable borrows, but it expresses the intent better, and MarkedArena now actually calls Context::do_collection.