Refactor process_message_3 to follow implementation guidelines #450

Job	Run time
check-style	13s
unit-tests (crypto-hacspec, ead-none)	1m 5s
unit-tests (crypto-hacspec, ead-zeroconf)	1m 5s
unit-tests (crypto-psa, ead-none)	1m 42s
unit-tests (crypto-psa, ead-zeroconf)	1m 12s
build-coap-example	1m 17s
build-edhoc-package (crypto-hacspec, ead-none)	2m 20s
build-static-lib-and-headers (crypto-psa-baremetal)	1m 20s
build-example-for-cortex-m4 (crypto-psa, ead-none)	1m 44s
build-edhoc-package (crypto-hacspec, ead-zeroconf)	1m 34s
build-static-lib-and-headers (crypto-cryptocell310)	1m 28s
build-example-for-cortex-m4 (crypto-psa, ead-zeroconf)	1m 10s
build-edhoc-package (crypto-psa, ead-none)	1m 53s
generate-fstar	0s
build-example-for-cortex-m4 (crypto-cryptocell310, ead-none)	1m 40s
build-edhoc-package (crypto-psa, ead-zeroconf)	1m 6s
build-example-for-cortex-m4 (crypto-cryptocell310, ead-zeroconf)	1m 35s
build-edhoc-package (crypto-psa-baremetal, ead-none)	1m 22s
run-example-on-qemu (crypto-psa, ead-none)	1m 40s
build-edhoc-package (crypto-psa-baremetal, ead-zeroconf)	1m 48s
run-example-on-qemu (crypto-psa, ead-zeroconf)	2m 16s
build-edhoc-package (crypto-cryptocell310, ead-none)	1m 15s
build-edhoc-package (crypto-cryptocell310, ead-zeroconf)	2m 12s
release	0s
	32m 57s

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