fix: separate any i/o to sequential device from caching layer

fix: separate any i/o to sequential device from caching layer
fix: a corner case on index resulting lookup fail in radix tree
fix: pass the incorrect argument to vfs_close when freeing up resources on destorying process
refactor: attach default file ops to inode for more flexible choice of ops when opening file
refactor: reduce the size of twifs_node.