Use a better approach for modular subtraction #6406

Job	Run time
Clang Tidy	1m 2s
Linux (gcc, shared)	2m 29s
macOS (shared, xcode, macos-13)	4m 13s
Linux (gcc, amalgamation)	4m 10s
Analysis (coverage, gcc, ubuntu-24.04)	30m 19s
macOS (amalgamation, xcode, macos-13)	4m 39s
Linux (gcc, static)	2m 14s
Analysis (sanitizer, clang, ubuntu-24.04)	30m 37s
Special (examples, gcc, ubuntu-24.04)	2m 19s
macOS (shared, xcode, macos-14)	2m 30s
Linux (clang, shared)	2m 46s
Analysis (valgrind, clang, ubuntu-24.04)	45m 14s
Special (minimized, gcc, ubuntu-24.04)	1m 32s
macOS (amalgamation, xcode, macos-14)	2m 52s
Special (bsi, gcc, ubuntu-24.04)	2m 1s
Analysis (fuzzers, gcc, ubuntu-24.04)	16m 42s
Analysis (lint, gcc, ubuntu-24.04)	53s
Special (docs, gcc, ubuntu-24.04)	59s
Windows (shared, x86_64, windows-2022)	7m 34s
Special (no_pcurves, gcc, ubuntu-24.04)	2m 44s
Analysis (format, gcc, ubuntu-24.04)	36s
Cross (cross-i386, gcc, ubuntu-22.04)	3m 50s
Windows (static, x86_64, windows-2022)	5m 56s
Analysis (limbo, gcc, ubuntu-24.04)	1m 23s
Cross (cross-arm32, gcc, ubuntu-24.04)	14m 37s
Windows (amalgamation, x86_64, windows-2022)	7m 47s
Cross (cross-arm64, gcc, ubuntu-24.04)	20m 2s
Windows (shared, x86, windows-2022)	9m 46s
Cross (cross-ppc64, gcc, ubuntu-24.04)	9m 20s
Cross (cross-mips64, gcc, ubuntu-24.04)	10m 44s
Cross (cross-android-arm64, clang, ubuntu-24.04)	6m 21s
Cross (static, gcc, windows-2022, make)	4m 19s
Cross (cross-ios-arm64, xcode, macos-13)	1m 19s
	4h 23m 49s

Provide feedback