This greatly simplifies mod loading, eliminates the .cache folders by loading assemblies in memory, ensures DLLs are loaded in leaf-to-root order, and reduces log verbosity. These changes should address a range of issues, notably #221 and #226.

This commit ensures DLLs are copied to the cache directory only if they changed, to avoid breaking debugging support unless necessary. To support this change, the assembly hash file has been replaced with a more detailed JSON structure, which is used to determine whether the cache is up-to-date and whether to use the cached or original assembly. Some mods contain multiple DLLs, which must be kept together to prevent assembly resolution issues; to simplify that (and avoid orphaned cache entries), each mod now has its own separate cache.