Improve field element halving #5531

Job	Run time
Clang Tidy	23s
Linux (gcc, shared)	2m 2s
Linux (gcc, amalgamation)	3m 49s
Linux (gcc, static)	1m 43s
Linux (clang, shared)	2m 6s
macOS (shared, xcode, macos-13)	4m 16s
Windows (shared, x86_64, windows-2022)	9m 1s
Windows (static, x86_64, windows-2022)	3m 48s
Analysis (coverage, gcc, ubuntu-24.04)	26m 0s
macOS (amalgamation, xcode, macos-13)	3m 53s
Special (examples, gcc, ubuntu-24.04)	47s
Windows (amalgamation, x86_64, windows-2022)	4m 52s
Analysis (sanitizer, msvc, windows-2022, ninja)	19m 46s
Analysis (sanitizer, clang, ubuntu-24.04)	11m 12s
Windows (shared, x86, windows-2022)	10m 56s
Special (minimized, gcc, ubuntu-24.04)	16s
macOS (shared, xcode, macos-14)	2m 22s
Analysis (sanitizer, gcc, ubuntu-24.04)	17m 43s
Special (bsi, gcc, ubuntu-24.04)	44s
Special (docs, gcc, ubuntu-24.04)	48s
macOS (amalgamation, xcode, macos-14)	2m 36s
Analysis (valgrind, clang, ubuntu-24.04)	20m 23s
Analysis (fuzzers, gcc, ubuntu-24.04)	6m 23s
Analysis (lint, gcc, ubuntu-24.04)	42s
Cross (cross-i386, gcc, ubuntu-22.04)	3m 9s
Analysis (format, gcc, ubuntu-24.04)	26s
Cross (cross-arm32, gcc, ubuntu-24.04)	9m 54s
Analysis (limbo, gcc, ubuntu-24.04)	1m 3s
Cross (cross-arm64, gcc, ubuntu-24.04)	6m 27s
Cross (cross-ppc64, gcc, ubuntu-24.04)	6m 6s
Cross (cross-mips64, gcc, ubuntu-24.04)	6m 33s
Cross (cross-android-arm64, clang, ubuntu-24.04)	2m 16s
Cross (static, gcc, windows-2022, make)	4m 33s
Cross (cross-ios-arm64, xcode, macos-13)	1m 2s
	3h 18m 0s

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