(core) Add RenameChoices user action

Summary:
["RenameChoices", table_id, col_id, renames]
Updates the data in a Choice/ChoiceList column to reflect the new choice names.
`renames` should be a dict of `{old_choice_name: new_choice_name}`.
This doesn't touch the choices configuration in widgetOptions, that must be done separately.
Frontend to be done in another diff.

Test Plan: Added two Python unit tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3050

sandbox/grist/column.py

@@ -225,6 +225,20 @@ class BaseColumn(object):
     # pylint: disable=no-self-use, unused-argument
     return values, []
 
+  def rename_choices(self, renames):
+    row_ids = []
+    values = []
+    for row_id, value in enumerate(self._data):
+      if value is not None and self.type_obj.is_right_type(value):
+        value = self._rename_cell_choice(renames, value)
+        if value is not None:
+          row_ids.append(row_id)
+          values.append(value)
+    return row_ids, values
+
+  def _rename_cell_choice(self, renames, value):
+    return renames.get(value, value)
+
 
 class DataColumn(BaseColumn):
   """
@@ -361,6 +375,9 @@ class ChoiceListColumn(BaseColumn):
   def _make_rich_value(self, typed_value):
     return () if typed_value is None else typed_value
 
+  def _rename_cell_choice(self, renames, value):
+    return tuple(renames.get(choice, choice) for choice in value)
+
 
 class BaseReferenceColumn(BaseColumn):
   """