Upgrade sundown to hoedown in rustdoc

https://github.com/hoedown/hoedown

Benefits include upstream being maintained, and correctly parsing codefences, e.g. parsing

``````
 ```
 a
 ~~~
 b
 ~~~
 c
 ```
``````

as a single nested code block

> ```
> a 
> ~~~
> b
> ~~~
> c
> ```

rather than two separate ones

> ```
> a
> ```
> 
> b
> 
> ```
> c
> ```
