Part of #43927. Split from #44575. The get_send() and get_receive() methods of the Variant class do their own formatting thing that needs to be replaced with something more robust. For this reason, they've been left out of a number of recent refactorings, which should also be applied to them once viable.

Steps to be done:

For Variant.get_receive(), along with Variant.get_delta_receive_body() (and Field.get_get_real()):
- #44597 Replace the <var>-based custom formatting with a string join
- #44598 Distribute the % (printf-style) formatting out to the individual parts (that need them) in get_receive()
- #44599 Replace % formatting with str.format() and only the required arguments in get_receive(); cf. #44575
- #44601 Deal with string formatting in get_delta_receive_body() (and Field.get_get_real())
For Variant.get_send(), along with Variant.get_delta_send_body()
- #44602 Give get_delta_send_body() access to pre2 (in some way) so that it doesn't need the custom formatting anymore
- #44603 Replace the <var>-based custom formatting with a string join
- #44604 Distribute the % (printf-style) formatting out to the individual parts (that need them) in get_send()
- #44605 Replace % formatting with str.format() and only the required arguments in get_send(); cf. #44575
- #44606 Deal with string formatting in get_delta_send_body()
#44607 Apply the equivalent of #44584 to the resulting code