P1001R2 execution::unseq

[StephanTLavavej]

P1001R2 execution::unseq

This may need support from the compiler back-end (tracked by Microsoft-internal VSO-1143256).

Feature-test macro as of WG21-N4842 (increased from current value; note that this value differs from the "suggestion" in WG21-P1001):
#define __cpp_lib_execution 201902L