add Column.len (#331)

Author: MarcoGorelli

spec/API_specification/dataframe_api/column_object.py

@@ -90,10 +90,6 @@ def name(self) -> str:
         """Return name of column."""
         ...
 
-    def __len__(self) -> int:
-        """Return the number of rows."""
-        ...
-
     def __iter__(self) -> NoReturn:
         """Iterate over elements.
 
@@ -750,6 +746,10 @@ def var(
         """
         ...
 
+    def len(self) -> Scalar:
+        """Return the number of rows."""
+        ...
+
     def n_unique(self, *, skip_nulls: bool = True) -> Scalar:
         """Return number of unique values.