fn: introduce fn::index as alias to positional (#1806) #1387

Job	Run time
perftests (gcc-8, release)	1m 36s
perftests (gcc-9, release)	1m 39s
perftests (gcc-10, release)	1m 38s
perftests (gcc-11, release)	1m 38s
perftests (gcc-12, debug)	2m 19s
perftests (gcc-12, release)	1m 40s
perftests (gcc-13, debug)	2m 9s
perftests (gcc-13, release)	1m 41s
perftests (clang-11, debug)	1m 52s
perftests (clang-11, release)	1m 57s
perftests (clang-12, debug)	1m 59s
perftests (clang-12, release)	1m 58s
perftests (clang-13, release)	2m 0s
perftests (clang-14, release)	2m 7s
perftests (clang-15, debug)	1m 50s
perftests (clang-15, release)	1m 52s
perftests (clang-16, debug)	1m 53s
perftests (clang-16, release)	1m 57s
perftests (clang-14-cuda-11, debug)	5m 52s
perftests (clang-14-cuda-11, release)	5m 5s
perftests (gcc-10-cuda-11.8, debug)	5m 39s
perftests (gcc-10-cuda-11.8, release)	5m 11s
perftests (gcc-11-cuda-12.0, debug)	5m 39s
perftests (gcc-11-cuda-12.0, release)	5m 2s
perftests (gcc-12-cuda-12.3, debug)	5m 58s
perftests (gcc-12-cuda-12.3, release)	5m 13s
perftests (gcc-12-cuda-12.4, debug)	6m 13s
perftests (gcc-12-cuda-12.4, release)	5m 20s
perftests (base-hip, debug)	7m 21s
perftests (base-hip, release)	6m 14s
perftests (gcc-10-hpx, release)	3m 33s
perftests (nvhpc-23.3, debug)	8m 53s
perftests (nvhpc-23.3, release)	10m 4s
perftests (nvhpc-23.9, debug)	13m 18s
perftests (nvhpc-23.9, release)	10m 31s
	2h 28m 51s

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