fn: introduce fn::index as alias to positional #1384

Job	Run time
perftests (gcc-8, release)	1m 34s
perftests (gcc-9, release)	1m 32s
perftests (gcc-10, release)	1m 46s
perftests (gcc-11, release)	1m 45s
perftests (gcc-12, debug)	2m 20s
perftests (gcc-12, release)	1m 43s
perftests (gcc-13, debug)	2m 6s
perftests (gcc-13, release)	1m 39s
perftests (clang-11, debug)	1m 50s
perftests (clang-11, release)	1m 51s
perftests (clang-12, debug)	1m 55s
perftests (clang-12, release)	1m 56s
perftests (clang-13, release)	2m 2s
perftests (clang-14, release)	2m 7s
perftests (clang-15, debug)	1m 48s
perftests (clang-15, release)	1m 56s
perftests (clang-16, debug)	1m 54s
perftests (clang-16, release)	2m 5s
perftests (clang-14-cuda-11, debug)	5m 52s
perftests (clang-14-cuda-11, release)	5m 9s
perftests (gcc-10-cuda-11.8, debug)	5m 43s
perftests (gcc-10-cuda-11.8, release)	5m 26s
perftests (gcc-11-cuda-12.0, debug)	5m 40s
perftests (gcc-11-cuda-12.0, release)	5m 3s
perftests (gcc-12-cuda-12.3, debug)	5m 56s
perftests (gcc-12-cuda-12.3, release)	5m 13s
perftests (gcc-12-cuda-12.4, debug)	5m 59s
perftests (gcc-12-cuda-12.4, release)	4m 57s
perftests (base-hip, debug)	7m 21s
perftests (base-hip, release)	6m 35s
perftests (gcc-10-hpx, release)	3m 34s
perftests (nvhpc-23.3, debug)	9m 13s
perftests (nvhpc-23.3, release)	10m 44s
perftests (nvhpc-23.9, debug)	9m 46s
perftests (nvhpc-23.9, release)	13m 12s
	2h 29m 12s

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