Traversing leaves is possible by any entity, but a slowdown is applied depending on the entity size.
Bigger entities move faster, since they are stronger (in theory). Vertical movement is unaffected by it,
because it's leaves duh, you can't just fly in them.
Leaves are added to #climbable tag to accompany this change.
This is based on the assumption of leaves having small branches inside like real trees.
Copying from the mixin doc: