clippy: disable the box_default lint

The clippy website explains that `Box::new(Default::default())` can be
shortened to `Box::default()` and that it's more readable. That's true…
when you don't have a generic parameter in the mix (which may be
required if rustc can't infer the type of the boxee), in which case it
just looks bad, e.g. `Box::<MyType>::default()` instead of
`Box::new(MyType::default())`.

I do strongly prefer the latter, and propose to get rid of the lint, as
a result.

.cargo/config.toml

@@ -38,6 +38,7 @@ rustflags = [
     "-Wclippy::nonstandard_macro_braces",
     "-Wclippy::str_to_string",
     "-Wclippy::todo",
+    "-Aclippy::box_default"
 ]
 
 [target.'cfg(target_arch = "wasm32")']