Bump `heapless` dependency to v0.8 #2063

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 18s
test (1.65.0)	5m 14s
build (1.65.0, wasm32-unknown-unknown)	5m 49s
test (stable)	20s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	9s
	17m 3s

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