clean up: organized imports
moved .fs and .db packages to new .io package
moved packages .function .plugin .builtin into new .lang package
moved FileSystemModule.SMOOTH_DIR to IoConstants
renamed HashedDbWithTasks -> TasksCache, HashedDbWithValues -> ValuesCache
created .lang.function.value and moved Hashed, Value interfaces and its subtypes there
renamed io.db -> io.cache
renamed XxxObject -> CachedXxx