Prep work for to_wkt (#795)

### Change list

- Convert geo-traits scalars to `wkt::Wkt`
- This is prep work for direct transformation of geoarrow scalars to
`wkt::Wkt` objects.
- This does not go through geozero, partially because I hit issues with
multidimensional writing in georust/geozero#237
- This supersedes #788,
though it hasn't yet piped through to a full `ToWkt` trait implemented
on arrays. cc @b4l

src/io/wkt/mod.rs

@@ -1 +1,2 @@
 pub mod reader;
+pub mod writer;

src/io/wkt/writer/mod.rs

@@ -0,0 +1 @@
+mod scalar;

src/io/wkt/writer/scalar.rs

@@ -0,0 +1,135 @@
+#![allow(dead_code)]
+use geo::CoordFloat;
+
+use crate::geo_traits::{
+    CoordTrait, GeometryCollectionTrait, GeometryTrait, GeometryType, LineStringTrait,
+    MultiLineStringTrait, MultiPointTrait, MultiPolygonTrait, PointTrait, PolygonTrait, RectTrait,
+};
+
+fn coord_to_wkt<T: CoordFloat>(coord: &impl CoordTrait<T = T>) -> wkt::types::Coord<T> {
+    let mut out = wkt::types::Coord {
+        x: coord.x(),
+        y: coord.y(),
+        z: None,
+        m: None,
+    };
+    if coord.dim() == 3 {
+        out.z = Some(coord.nth_unchecked(2));
+    }
+    out
+}
+
+fn point_to_wkt<T: CoordFloat>(point: &impl PointTrait<T = T>) -> wkt::types::Point<T> {
+    if point.x().is_nan() && point.y().is_nan() {
+        return wkt::types::Point(None);
+    }
+
+    let mut coord = wkt::types::Coord {
+        x: point.x(),
+        y: point.y(),
+        z: None,
+        m: None,
+    };
+    if point.dim() == 3 {
+        coord.z = Some(point.nth_unchecked(2));
+    }
+    wkt::types::Point(Some(coord))
+}
+
+fn line_string_to_wkt<T: CoordFloat>(
+    line_string: &impl LineStringTrait<T = T>,
+) -> wkt::types::LineString<T> {
+    wkt::types::LineString(
+        line_string
+            .coords()
+            .map(|coord| coord_to_wkt(&coord))
+            .collect(),
+    )
+}
+
+fn polygon_to_wkt<T: CoordFloat>(polygon: &impl PolygonTrait<T = T>) -> wkt::types::Polygon<T> {
+    let mut rings = vec![];
+    if let Some(exterior) = polygon.exterior() {
+        rings.push(line_string_to_wkt(&exterior));
+    }
+    polygon
+        .interiors()
+        .for_each(|interior| rings.push(line_string_to_wkt(&interior)));
+    wkt::types::Polygon(rings)
+}
+
+fn multi_point_to_wkt<T: CoordFloat>(
+    multi_point: &impl MultiPointTrait<T = T>,
+) -> wkt::types::MultiPoint<T> {
+    wkt::types::MultiPoint(
+        multi_point
+            .points()
+            .map(|point| point_to_wkt(&point))
+            .collect(),
+    )
+}
+
+fn multi_line_string_to_wkt<T: CoordFloat>(
+    multi_line_string: &impl MultiLineStringTrait<T = T>,
+) -> wkt::types::MultiLineString<T> {
+    wkt::types::MultiLineString(
+        multi_line_string
+            .lines()
+            .map(|line| line_string_to_wkt(&line))
+            .collect(),
+    )
+}
+
+fn multi_polygon_to_wkt<T: CoordFloat>(
+    multi_polygon: &impl MultiPolygonTrait<T = T>,
+) -> wkt::types::MultiPolygon<T> {
+    wkt::types::MultiPolygon(
+        multi_polygon
+            .polygons()
+            .map(|polygon| polygon_to_wkt(&polygon))
+            .collect(),
+    )
+}
+
+/// Convert any Rect to a [`geo::Rect`].
+///
+/// Only the first two dimensions will be kept.
+fn rect_to_wkt<T: CoordFloat>(_rect: &impl RectTrait<T = T>) -> wkt::types::Polygon<T> {
+    todo!()
+    // Need to create custom coords for a polygon box, see
+    // https://github.com/georust/geo/blob/68f80f851879dd58f146aae47dc2feeea6c83230/geo-types/src/geometry/rect.rs#L217-L225
+}
+
+/// Convert any Geometry to a [`geo::Geometry`].
+///
+/// Only the first two dimensions will be kept.
+fn geometry_to_wkt<T: CoordFloat>(geometry: &impl GeometryTrait<T = T>) -> wkt::Wkt<T> {
+    match geometry.as_type() {
+        GeometryType::Point(geom) => wkt::Wkt::Point(point_to_wkt(geom)),
+        GeometryType::LineString(geom) => wkt::Wkt::LineString(line_string_to_wkt(geom)),
+        GeometryType::Polygon(geom) => wkt::Wkt::Polygon(polygon_to_wkt(geom)),
+        GeometryType::MultiPoint(geom) => wkt::Wkt::MultiPoint(multi_point_to_wkt(geom)),
+        GeometryType::MultiLineString(geom) => {
+            wkt::Wkt::MultiLineString(multi_line_string_to_wkt(geom))
+        }
+        GeometryType::MultiPolygon(geom) => wkt::Wkt::MultiPolygon(multi_polygon_to_wkt(geom)),
+        GeometryType::GeometryCollection(geom) => {
+            wkt::Wkt::GeometryCollection(geometry_collection_to_wkt(geom))
+        }
+        GeometryType::Rect(geom) => wkt::Wkt::Polygon(rect_to_wkt(geom)),
+    }
+}
+
+/// Convert any GeometryCollection to a [`geo::GeometryCollection`].
+///
+/// Only the first two dimensions will be kept.
+fn geometry_collection_to_wkt<T: CoordFloat>(
+    geometry_collection: &impl GeometryCollectionTrait<T = T>,
+) -> wkt::types::GeometryCollection<T> {
+    wkt::types::GeometryCollection(
+        geometry_collection
+            .geometries()
+            .map(|geometry| geometry_to_wkt(&geometry))
+            .collect(),
+    )
+}