Instead of passing clones of context and the markdown tree to
postprocessors, pass them a mutable reference which may be modified
in-place.
This is a breaking change to the postprocessor implementation, changing
both the input arguments as well as the return value:
```diff
- dyn Fn(Context, MarkdownEvents) -> (Context, MarkdownEvents, PostprocessorResult) + Send + Sync;
+ dyn Fn(&mut Context, &mut MarkdownEvents) -> PostprocessorResult + Send + Sync;
```
With this change the postprocessor API becomes a little more ergonomic
to use however, especially making the intent around return statements more clear.
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