P0768R1 Library Support For The Spaceship Comparison Operator <=>
P1959R0 Removing weak_equality And strong_equality

This was partially implemented in VS 2019 16.0, but some work remains to be done.

This deprecates rel_ops.

Feature-test macro added by WG21-P1353 "Missing Feature Test Macros" (note that this value will be increased by #62 WG21-P1614 "Adding Spaceship <=> To The Library"):
#define __cpp_lib_three_way_comparison 201711L