Fix broken links (mainmatter#47)

Author: Sympatron

book/src/02_basic_calculator/01_integers.md

@@ -117,7 +117,7 @@ error[E0308]: mismatched types
   |
 ```
 
-We'll see how to convert between types [later in this course](../04_traits/08_from).
+We'll see how to convert between types [later in this course](../04_traits/09_from).
 
 ## References
 
@@ -134,5 +134,5 @@ behave.
 We'll talk about operator overloading [later in the course](../04_traits/03_operator_overloading), after we've covered traits.
 
 [^coercion]: There are some exceptions to this rule, mostly related to references, smart pointers and ergonomics. We'll
-cover those [later on](../04_traits/06_deref).
+cover those [later on](../04_traits/07_deref).
 A mental model of "all conversions are explicit" will serve you well in the meantime.

book/src/02_basic_calculator/04_panics.md

@@ -1,6 +1,6 @@
 # Panics
 
-Let's go back to the `speed` function you wrote for the ["Variables" section](../02_variables/README.md).
+Let's go back to the `speed` function you wrote for the ["Variables" section](02_variables).
 It probably looked something like this:
 
 ```rust

book/src/02_basic_calculator/10_as_casting.md

@@ -92,7 +92,7 @@ It is also fairly limited: you can only rely on `as` casting
 for primitive types and a few other special cases.  
 When working with composite types, you'll have to rely on 
 different conversion mechanisms ([fallible](../05_ticket_v2/13_try_from) 
-and [infallible](../04_traits/08_from)), which we'll explore later on.
+and [infallible](../04_traits/09_from)), which we'll explore later on.
 
 ## References
 

book/src/03_ticket_v1/05_encapsulation.md

@@ -38,7 +38,7 @@ let ticket = Ticket {
 You've seen this in action in the previous exercise on visibility.  
 We now need to provide one or more public **constructors**—i.e. static methods or functions that can be used
 from outside the module to create a new instance of the struct.  
-Luckily enough we already have one: `Ticket::new`, as implemented in [a previous exercise](../02_validation/README.md).
+Luckily enough we already have one: `Ticket::new`, as implemented in [a previous exercise](02_validation).
 
 ## Accessor methods
 

book/src/03_ticket_v1/10_references_in_memory.md

@@ -49,6 +49,6 @@ They just point to a memory location, which _may_ be on the heap, but doesn't ha
 
 - The exercise for this section is located in `exercises/03_ticket_v1/10_references_in_memory`
 
-[^fat]: [Later in the course](../04_traits/05_str_slice) we'll talk about **fat pointers**,
+[^fat]: [Later in the course](../04_traits/06_str_slice) we'll talk about **fat pointers**,
 i.e. pointers with additional metadata. As the name implies, they are larger than
 the pointers we discussed in this chapter, also known as **thin pointers**.

book/src/04_traits/08_sized.md

@@ -6,7 +6,7 @@ From our previous [discussion on memory layouts](../03_ticket_v1/10_references_i
 it would have been reasonable to expect `&str` to be represented as a single `usize` on
 the stack, a pointer. That's not the case though. `&str` stores some **metadata** next 
 to the pointer: the length of the slice it points to. Going back to the example from 
-[a previous section](05_str_slice.md):
+[a previous section](06_str_slice):
 
 ```rust
 let mut s = String::with_capacity(5);

book/src/05_ticket_v2/09_error_trait.md

@@ -22,7 +22,7 @@ that implements the `Error` trait.
 pub trait Error: Debug + Display {}
 ```
 
-You might recall the `:` syntax from [the `Sized` trait](../04_traits/07_sized.md)—it's used to specify **supertraits**.
+You might recall the `:` syntax from [the `Sized` trait](../04_traits/08_sized.md)—it's used to specify **supertraits**.
 For `Error`, there are two supertraits: `Debug` and `Display`. If a type wants to implement `Error`, it must also
 implement `Debug` and `Display`.
 

book/src/05_ticket_v2/13_try_from.md

@@ -1,6 +1,6 @@
 # `TryFrom` and `TryInto`
 
-In the previous chapter we looked at the [`From` and `Into` traits](../04_traits/08_from.md), 
+In the previous chapter we looked at the [`From` and `Into` traits](../04_traits/09_from.md), 
 Rust's idiomatic interfaces for **infallible** type conversions.  
 But what if the conversion is not guaranteed to succeed?