Improve field count typical case performance #381

Job	Run time
windows (msvc-14.0, 14,latest, 32,64, windows-2019, -j3)	1m 29s
posix (gcc-14, 03,11,14,17,20, ubuntu-24.04)	7m 14s
windows (gcc, 03,11,14,17,2a, 64, windows-2019, -j3)	7m 12s
posix (gcc-12, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	5m 43s
windows (msvc-14.3, 20,latest, 64, windows-2022, -j1)	14m 32s
posix (gcc-11, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	6m 9s
windows (clang-win, 14,17,2a,latest, 32,64, windows-2022)	17m 17s
posix (gcc-10, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	6m 20s
posix (gcc-9, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefined...	5m 18s
posix (clang-15, 03,11,14,17,20, ubuntu-22.04)	6m 31s
finish	0s
	1h 17m 45s

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