The `asm!` macro is not allowed in naked functions now

[yct21]

After this PR merged in nightly toolchain, asm! macro is not allowed in naked functions. This affects code in chapter 5.

~/code/Asynchronous-Programming-in-Rust/ch05/c-fibers > cargo +nightly run                              
   Compiling c-fibers v0.1.0 (/home/yct21/code/Asynchronous-Programming-in-Rust/ch05/c-fibers)
error[E0787]: the `asm!` macro is not allowed in naked functions
   --> src/main.rs:144:5
    |
144 |     asm!("ret", options(noreturn))
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ consider using the `naked_asm!` macro instead

error[E0787]: the `asm!` macro is not allowed in naked functions
   --> src/main.rs:158:5
    |
158 | /     asm!(
159 | |         "mov [rdi + 0x00], rsp",
160 | |         "mov [rdi + 0x08], r15",
161 | |         "mov [rdi + 0x10], r14",
...   |
174 | |         options(noreturn)
175 | |     );
    | |_____^ consider using the `naked_asm!` macro instead

[cfsamson]

Thank you for the heads up. I've updated the examples in the repository so it compiles correctly.