Add result_descriptions to Rows and Row

This can be useful for functions that are just passed a `Rows` or `Row`
and want to introspect a bit into the columns.

src/lib.rs

@@ -1364,6 +1364,11 @@ impl<'stmt> Rows<'stmt> {
         self.read_rows()
     }
 
+    /// Returns a slice describing the columns of the `Rows`.
+    pub fn result_descriptions(&self) -> &'stmt [ResultDescription] {
+        self.stmt.result_descriptions()
+    }
+
     /// Consumes the `Rows`, cleaning up associated state.
     ///
     /// Functionally identical to the `Drop` implementation on `Rows`
@@ -1416,6 +1421,11 @@ impl<'stmt> Row<'stmt> {
         self.data.len()
     }
 
+    /// Returns a slice describing the columns of the `Row`.
+    pub fn result_descriptions(&self) -> &'stmt [ResultDescription] {
+        self.stmt.result_descriptions()
+    }
+
     /// Retrieves the contents of a field of the row.
     ///
     /// A field can be accessed by the name or index of its column, though