* We no longer maintain the weird `loadingAssemblies` global state.
* AssemblyList now internally handles multiple concurrent load requests for the same filename.
* AssemblyList.assemblies and its lock is now private to the AssemblyList.
* Removed a questionable caching layer (cache was per-AssemblyList, but was caching the result of a per-LoadedAssembly lookup function.
* Replaced static DisableAssemblyLoad() with bool-parameter on GetAssemblyResolver() call.