Fix CI nightly's mismatched_lifetime_syntaxes

[ThierryBerger]

Current master has warnings on nightly due to a bunch of mismatched_lifetime_syntaxes:

warning: lifetime flowing from input to output with different syntax can be confusing
  --> crates/parry3d/../../src/partitioning/bvh/bvh_traverse.rs:79:44
   |
79 |     pub fn leaves<F: Fn(&BvhNode) -> bool>(&self, check_node: F) -> Leaves<F> {
   |                                            ^^^^^                    --------- the lifetime gets resolved as `'_`
   |                                            |
   |                                            this lifetime flows to the output
   |
   = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: one option is to remove the lifetime for references and use the anonymous lifetime for paths
   |
79 |     pub fn leaves<F: Fn(&BvhNode) -> bool>(&self, check_node: F) -> Leaves<'_, F> {
   |

[sebcrozet]

Thanks!