(core) Improve object serialization, to help get RECORD data to Custom Widgets.

Summary:
- Change RECORD's dates_as_str default to False.
- Reimplement objtype encode_object/decode_object with less machinery.
- Implement encoding of dicts (with string keys).
- Make lists and dicts encode values recursively.
- Implement encoding/decoding in the client
- Decode automatically in plugins' fetchSelectedTable/Record, with an option to skip.

Test Plan: Tested manually, not sure what tests may be affected yet.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2593

sandbox/grist/functions/info.py

@@ -500,13 +500,12 @@ def CELL(info_type, reference):
   raise NotImplementedError()
 
 
-def RECORD(record_or_list, dates_as_iso=True, expand_refs=0):
+def RECORD(record_or_list, dates_as_iso=False, expand_refs=0):
   """
   Returns a Python dictionary with all fields in the given record. If a list of records is given,
   returns a list of corresponding Python dictionaries.
 
-  If dates_as_iso is set (which is the default), Date and DateTime values are converted to string
-  using ISO 8601 format.
+  If dates_as_iso is set, Date and DateTime values are converted to string using ISO 8601 format.
 
   If expand_refs is set to 1 or higher, Reference values are replaced with a RECORD representation
   of the referenced record, expanding the given number of levels.
@@ -540,7 +539,7 @@ def RECORD(record_or_list, dates_as_iso=True, expand_refs=0):
           for r in records]
 
 
-def _prepare_record_dict(record, dates_as_iso=True, expand_refs=0):
+def _prepare_record_dict(record, dates_as_iso=False, expand_refs=0):
   table_id = record._table.table_id
   docmodel = record._table._engine.docmodel
   columns = docmodel.get_table_rec(table_id).columns