Improve field count typical case performance (#120) #390

Job	Run time
windows (msvc-14.0, 14,latest, 32,64, windows-2019, -j3)	3m 57s
windows (gcc, 03,11,14,17,2a, 64, windows-2019, -j3)	13m 34s
posix (gcc-14, 03,11,14,17,20, ubuntu-24.04)	7m 26s
windows (msvc-14.3, 20,latest, 64, windows-2022, -j1)	15m 51s
posix (gcc-12, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	6m 57s
windows (clang-win, 14,17,2a,latest, 32,64, windows-2022)	18m 13s
posix (gcc-11, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	8m 22s
posix (gcc-10, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	10m 41s
posix (gcc-9, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefined...	8m 32s
posix (clang-15, 03,11,14,17,20, ubuntu-22.04)	8m 12s
finish	2s
	1h 41m 47s

Provide feedback