Fix message framing (backtracking parser) #441

Job	Run time
formatting	4s
mavlink-dump	51s
linting	1m 36s
internal-tests (ardupilotmega)	1m 1s
internal-tests (asluav)	1m 16s
msrv	5m 9s
internal-tests (matrixpilot)	1m 17s
internal-tests (minimal)	1m 10s
internal-tests (paparazzi)	1m 21s
internal-tests (python_array_test)	1m 14s
internal-tests (slugs)	1m 8s
internal-tests (standard)	1m 7s
internal-tests (test)	1m 7s
internal-tests (ualberta)	1m 17s
internal-tests (uavionix)	1m 3s
internal-tests (icarous)	1m 4s
internal-tests (common)	1m 6s
docs	28s
build (macos-latest, x86_64-apple-darwin, --features ardupilotmega)	5m 27s
build (ubuntu-latest, arm-unknown-linux-musleabihf, --features ardupilotmega)	2m 2s
build (ubuntu-latest, armv7-unknown-linux-musleabihf, --features ardupilotmega)	2m 0s
build (ubuntu-latest, x86_64-unknown-linux-musl, --features ardupilotmega)	2m 4s
build (ubuntu-latest, x86_64-unknown-linux-musl, --features ardupilotmega,emit-description,emit-e...	2m 10s
build (ubuntu-latest, thumbv7m-none-eabi, --no-default-features --features embedded)	1m 22s
build (windows-latest, x86_64-pc-windows-msvc, --features ardupilotmega)	3m 23s
test-embedded-size	0s
	41m 47s

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