Add custom types and simplify highway query

layers/highway/ne_global_roads.sql

@@ -0,0 +1,23 @@
+CREATE OR REPLACE FUNCTION ne_highway(type VARCHAR) RETURNS VARCHAR AS $$
+  SELECT CASE type
+    WHEN 'Major Highway' THEN 'motorway'
+    WHEN 'Secondary Highway' THEN 'trunk'
+    WHEN 'Road' THEN 'primary'
+    ELSE type
+  END;
+$$ LANGUAGE SQL IMMUTABLE;
+
+CREATE TABLE IF NOT EXISTS ne_10m_global_roads AS (
+    SELECT geom AS geometry, scalerank, ne_highway(type) AS highway
+    FROM ne_10m_roads
+    WHERE continent <> 'North America'
+      AND featurecla = 'Road'
+      AND type IN ('Major Highway', 'Secondary Highway', 'Road')
+    UNION ALL
+    SELECT geom AS geometry, scalerank, ne_highway(type) AS highway
+    FROM ne_10m_roads_north_america
+    WHERE type IN ('Major Highway', 'Secondary Highway', 'Road')
+);
+
+CREATE INDEX IF NOT EXISTS ne_10m_global_roads_geometry_idx ON ne_10m_global_roads USING gist(geometry);
+CREATE INDEX IF NOT EXISTS ne_10m_global_roads_scalerank_idx ON ne_10m_global_roads(scalerank);