refactor: remove naked_functions nightly usage and filter packed apps to *.rs (#171)

Move the stackful coroutine context-switch implementation from naked Rust code into a standalone assembly file under user/src/bin, and update easy-fs-fuse to only treat .rs entries as user programs when packing fs.img. The packing behavior changed because keeping the assembly helper in user/src/bin would otherwise make the old directory scan misclassify .S files as runnable apps.