Unify Densify trait across metric spaces

Just like #1228, but for Densify rather than Length.

This enables Geodesic, Haversine, Euclidean, and Rhumb Densification.

This required implementing InterpolatePoint for Euclidean.

Adjacent work:

deprecated legacy DensifyHaversine
linestring_segment is now implemented in terms of the new Densify

NOTE: linestring_segment would be a good future candidate for
a similar unification across all the metric spaces

geo/src/algorithm/densify_haversine.rs

@@ -9,6 +9,10 @@ use crate::{
     Triangle,
 };
 
+#[deprecated(
+    since = "0.29.0",
+    note = "Please use the `line.densify::<Haversine>()` via the `Densify` trait instead."
+)]
 /// Returns a new spherical geometry containing both existing and new interpolated coordinates with
 /// a maximum distance of `max_distance` between them.
 ///
@@ -21,6 +25,7 @@ use crate::{
 /// # Examples
 /// ```
 /// use geo::{coord, Line, LineString};
+/// #[allow(deprecated)]
 /// use geo::DensifyHaversine;
 ///
 /// let line = Line::new(coord! {x: 0.0, y: 0.0}, coord! { x: 0.0, y: 1.0 });
@@ -218,6 +223,7 @@ mod tests {
         ]
         .into();
 
+        #[allow(deprecated)]
         let dense = polygon.densify_haversine(50000.0);
         assert_relative_eq!(dense.exterior(), &output_exterior);
     }
@@ -250,6 +256,7 @@ mod tests {
         ]
         .into();
 
+        #[allow(deprecated)]
         let dense = linestring.densify_haversine(110.0);
         assert_relative_eq!(dense, output);
     }
@@ -258,13 +265,15 @@ mod tests {
     fn test_line_densify() {
         let output: LineString = vec![[0.0, 0.0], [0.0, 0.5], [0.0, 1.0]].into();
         let line = Line::new(coord! {x: 0.0, y: 0.0}, coord! { x: 0.0, y: 1.0 });
+        #[allow(deprecated)]
         let dense = line.densify_haversine(100000.0);
         assert_relative_eq!(dense, output);
     }
 
     #[test]
     fn test_empty_linestring() {
         let linestring: LineString<f64> = LineString::new(vec![]);
+        #[allow(deprecated)]
         let dense = linestring.densify_haversine(10.0);
         assert_eq!(0, dense.coords_count());
     }