Investigate optimizing conversion of SPL tuple to/from dictionary.

[ddebrunner]

• Look at returning the dict to the C++ operators and convert it to a tuple using Python C-API.
  • - @spl.map @spl.source - Convert dict tuples directly to SPL tuples for spl.map and spl.source. #1562
  • - @spl.primitive - Return dict directly to C++ operator for @spl.primitive #1583
  • - Functional map, flat_map, source
  • - Functional aggregate
• Look at creating a default dict and use that as the base when converting SPL tuple to Python dict. May avoid resizing of the dict as it will already have the correct number of keys.

[ddebrunner]

#1562 added handling returned dict in the cgt code and improved performance for both tuple and dict as there is no longer one additional Python call layer.

The change for #1562 still needs to be appled to @spl.primitive and functional operators.

[markheger]

flat_map is still an open issue and covered in issue: #2486