Add missing core::panic::PanicInfo imports in code snippets (#440)

blog/content/second-edition/posts/01-freestanding-rust-binary/index.md

@@ -200,6 +200,8 @@ To tell the Rust compiler that we don't want to use the normal entry point chain
 #![no_std]
 #![no_main]
 
+use core::panic::PanicInfo;
+
 /// This function is called on panic.
 #[panic_implementation]
 #[no_mangle]
@@ -313,6 +315,8 @@ A minimal freestanding Rust binary looks like this:
 #![no_std] // don't link the Rust standard library
 #![no_main] // disable all Rust-level entry points
 
+use core::panic::PanicInfo;
+
 /// This function is called on panic.
 #[panic_implementation]
 #[no_mangle]

blog/content/second-edition/posts/02-minimal-rust-kernel/index.md

@@ -201,6 +201,8 @@ Compiling for our new target will use Linux conventions (I'm not quite sure why,
 #![no_std] // don't link the Rust standard library
 #![no_main] // disable all Rust-level entry points
 
+use core::panic::PanicInfo;
+
 /// This function is called on panic.
 #[panic_implementation]
 #[no_mangle]

blog/content/second-edition/posts/04-unit-testing/index.md

@@ -45,6 +45,8 @@ The problem is that unit tests are built for the host machine, with the `std` li
 ```rust
 // in src/main.rs
 
+use core::panic::PanicInfo;
+
 #[cfg(not(test))] // only compile when the test flag is not set
 #[panic_implementation]
 #[no_mangle]