mono collector: Reduce # of locking while walking the graph While profiling Zed's dev build I've noticed that while most of the time `upstream_monomorphizations` takes a lot of time in monomorpization_collector, in some cases (e.g. build of `editor` itself) the rest of monomorphization_collector_graph_walk dominates it. Most of the time is spent in collect_items_rec. This PR aims to reduce the number of locks taking place; instead of locking output MonoItems once per children of current node, we do so once per *current node*. We also get to reuse locks for mentioned and used items. While this commit does not reduce Wall time of Zed's build, it does shave off CPU time (measured with `cargo build -j1`) from 48s to 47s. I've also tested it with parallel frontend against Zed and ripgrep and found no regressions.