tls_codec: implement `U24` (#1284) #460

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 6s
build (1.65.0, wasm32-unknown-unknown)	1m 43s
test (1.65.0)	1m 51s
build (stable, thumbv7em-none-eabi)	23s
test (stable)	24s
build (stable, wasm32-unknown-unknown)	25s
minimal-versions / minimal-versions	30s
	7m 22s

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