Rfc 2070 panic implementation deprecated (#467)

* Make changes to code examples.

* Explain that panic_implementation has been deprecated

* Update attributes in source code.

src/main.rs

@@ -1,4 +1,3 @@
-#![feature(panic_implementation)] // required for defining the panic handler
 #![feature(abi_x86_interrupt)]
 #![no_std] // don't link the Rust standard library
 #![cfg_attr(not(test), no_main)] // disable all Rust-level entry points
@@ -35,7 +34,7 @@ pub extern "C" fn _start() -> ! {
 
 /// This function is called on panic.
 #[cfg(not(test))]
-#[panic_implementation]
+#[panic_handler]
 #[no_mangle]
 pub fn panic(info: &PanicInfo) -> ! {
     println!("{}", info);