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 *parent*. 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 `cargo build -j1` from 43s to 41.5s.

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