Minor refactoring of BoundingBox: change Eigen point accessor from

indices to .x(), .y(), .z()
Added Polyline vector accessors.
Polished Point hash code.

src/libslic3r/Polyline.hpp

@@ -61,6 +61,9 @@ public:
         }
     }
   
+    Point& operator[](Points::size_type idx) { return this->points[idx]; }
+    const Point& operator[](Points::size_type idx) const { return this->points[idx]; }
+
     const Point& last_point() const override { return this->points.back(); }
     const Point& leftmost_point() const;
     Lines lines() const override;