Chapter 07 - Damage get_available_exits() function errors

Hello,

I am working through the tutorial you have online for Rust and Rltk. I am on Chapter 07 - Damage and was hitting an error when you had us add
```
fn get_available_exits(&self, idx:usize) -> Vec<(usize, f32)> {
let mut exits : Vec<(usize, f32)> = Vec::new();
let x = idx as i32 % self.width;
let y = idx as i32 / self.width;
let w = self.width as usize;

// Cardinal directions
if self.is_exit_valid(x-1, y) { exits.push((idx-1, 1.0)) };
if self.is_exit_valid(x+1, y) { exits.push((idx+1, 1.0)) };
if self.is_exit_valid(x, y-1) { exits.push((idx-w, 1.0)) };
if self.is_exit_valid(x, y+1) { exits.push((idx+w, 1.0)) };

// Diagonals
if self.is_exit_valid(x-1, y-1) { exits.push(((idx-w)-1, 1.45)); }
if self.is_exit_valid(x+1, y-1) { exits.push(((idx-w)+1, 1.45)); }
if self.is_exit_valid(x-1, y+1) { exits.push(((idx+w)-1, 1.45)); }
if self.is_exit_valid(x+1, y+1) { exits.push(((idx+w)+1, 1.45)); }

exits
}
```

To the BaseMap impl. The error I was receiving was:

```
error[E0053]: method `get_available_exits` has an incompatible type for trait
--> src/map.rs:117:5
|
117 | fn get_available_exits(&self, idx:usize) -> Vec<(usize, f32)> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `smallvec::SmallVec`, found struct `std::vec::Vec`
|
= note: expected fn pointer `fn(&map::Map, _) -> smallvec::SmallVec<[(usize, f32); 10]>`
found fn pointer `fn(&map::Map, _) -> std::vec::Vec<(usize, f32)>`
```

After looking over the source code you provide I found that instead of using Vec like you have in the tutorial you used rltk::SmallVec. After changing my code to use rltk::SmallVec the error went away and I was getting the expected monster behavior.

Just wanted to bring this up in case others have hit this issue.