Add other river-like water area features to class=river (#1467)

Currently, river areas (`natural=water` + `water=river`) are rendered in the tiles as `class=river`.  However, canal, stream, ditch, and drain water areas are rendered as `class=lake`.  Since these types of objects are all flowing water, they should be grouped together into `class=river`, which allows for styles that render flowing water differently to do so on a consistent basis.

layers/water/mapping.yaml

@@ -107,6 +107,10 @@ tables:
       - dock
       water:
       - river
+      - stream
+      - canal
+      - ditch
+      - drain
       - pond
       - basin
       - wastewater