AP_Math: move zeroing to header, use memset, reuse in identity #43293

Job	Run time
build (stm32f7, chibios, 10)	14m 20s
build (stm32h7, chibios, 10)	12m 44s
build (fmuv2-plane, chibios, 10)	7m 6s
build (periph-build, chibios, 10)	11m 2s
build (iofirmware, chibios, 10)	4m 8s
build (CubeOrange-bootloader, chibios, 10)	3m 11s
build (fmuv3-bootloader, chibios, 10)	3m 0s
build (revo-bootloader, chibios, 10)	3m 2s
build (stm32h7-debug, chibios, 10)	8m 30s
build (fmuv3, chibios, 10)	15m 4s
build (revo-mini, chibios, 10)	11m 45s
build (MatekF405-Wing, chibios, 10)	12m 26s
build (CubeOrange-ODID, chibios, 10)	9m 34s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 6s
build (configure-all, chibios, 10)	12m 53s
build (build-options-defaults-test, chibios, 10)	23m 12s
build (signing, chibios, 10)	9m 49s
build (CubeOrange-PPP, chibios, 10)	8m 13s
build (CubeRed-EKF2, chibios, 10)	8m 4s
build (SOHW, chibios, 10)	11m 51s
build (Pixhawk6X-PPPGW, chibios, 10)	4m 5s
build (new-check, chibios, 10)	14m 10s
	3h 31m 15s

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