Bump `heapless` dependency to v0.8 (#1398) #1789

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 21s
test (1.65.0)	5m 23s
build (1.65.0, wasm32-unknown-unknown)	3m 32s
test (stable)	23s
minimal-versions	48s
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	10s
	15m 52s

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