Prefer explicit features for optional deps. (#681)

Implicit features are slated to be removed in a future version
of Rust (2024 edition).

Fixing this exposed 2 instances where the wrong feature was being
checked for `serde` vs `serde-serialize`.

src/lib.rs

@@ -28,7 +28,7 @@ pub extern crate parry3d_f64 as parry;
 
 pub extern crate crossbeam;
 pub extern crate nalgebra as na;
-#[cfg(feature = "serde")]
+#[cfg(feature = "serde-serialize")]
 #[macro_use]
 extern crate serde;
 extern crate num_traits as num;