(core) New type conversion in the backend

Summary: This is https://phab.getgrist.com/D3205 plus some changes (https://github.com/dsagal/grist/compare/type-convert...type-convert-server?expand=1) that move the conversion process to the backend. A new user action ConvertFromColumn uses `call_external` so that the data engine can delegate back to ActiveDoc. Code for creating formatters and parsers is significantly refactored so that most of the logic is in `common` and can be used in different ways.

Test Plan: The original diff adds plenty of tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3240

sandbox/grist/functions/info.py

@@ -500,6 +500,14 @@ def N(value):
   return 0
 
 
+def CURRENT_CONVERSION(rec):
+  """
+  Special function used only when changing the type of a column.
+  Doesn't work in normal formulas.
+  """
+  return rec.gristHelper_Converted
+
+
 def NA():
   """
   Returns the "value not available" error, `#N/A`.