Gmoshkin/reachability manager yield safety

Summary

• test: make test_large_snapshot less flaky

• fix: used to hold non-yield-safe references across yields

• refactor: yield-safe InstanceReachabilityManager

• feat: NoYieldsRefCell for yield-safe borrow checking

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here