Bump `heapless` dependency to v0.8 #1703

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 40s
build (1.65.0, wasm32-unknown-unknown)	3m 33s
build (stable, thumbv7em-none-eabi)	33s
minimal-versions	30s
test (1.65.0)	5m 30s
build (stable, wasm32-unknown-unknown)	31s
test (stable)	20s
	14m 37s

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