Fix line lengths

Author: Aaron1011

src/panic-implementation.md

@@ -4,7 +4,8 @@
 
 There are actually two panic macros - one defined in `libcore`, and one defined in `libstd`.
 This is due to the fact that code in `libcore` can panic. `libcore` is built before `libstd`,
-but we want panics to use the same machinery at runtime, whether they originate in `libcore` or `libstd`.
+but we want panics to use the same machinery at runtime, whether they originate in `libcore`
+or `libstd`.
 
 ##### libcore definition of panic!
 
@@ -65,12 +66,14 @@ we call ```__rust_start_panic```, which is provided by the panic runtime.
 The call to ```__rust_start_panic``` is very weird - it is passed a ```*mut &mut dyn BoxMeUp```,
 converted to an `usize`. Let's break this type down:
 
-1. `BoxMeUp` is an internal trait. It is implemented for `PanicPayload` (a wrapper around the user-supplied
-payload type), and has a method ```fn box_me_up(&mut self) -> *mut (dyn Any + Send)```.
+1. `BoxMeUp` is an internal trait. It is implemented for `PanicPayload`
+(a wrapper around the user-supplied payload type), and has a method
+```fn box_me_up(&mut self) -> *mut (dyn Any + Send)```.
 This method takes the user-provided payload (`T: Any + Send`), boxes it, and convertes the box to a raw pointer.
 
-2. When we call ```__rust_start_panic```, we have an `&mut dyn BoxMeUp`. However, this is a fat pointer
-(twice the size of a `usize`). To pass this to the panic runtime across an FFI boundary, we take a mutable
+2. When we call ```__rust_start_panic```, we have an `&mut dyn BoxMeUp`.
+However, this is a fat pointer (twice the size of a `usize`).
+To pass this to the panic runtime across an FFI boundary, we take a mutable
 reference *to this mutable reference* (`&mut &mut dyn BoxMeUp`), and convert it to a raw pointer
 (`*mut &mut dyn BoxMeUp`). The outer raw pointer is a thin pointer, since it points to a `Sized`
 type (a mutable reference). Therefore, we can convert this thin pointer into a `usize`, which