Bump `heapless` dependency to v0.8 #2435

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	7m 8s
build (1.65.0, wasm32-unknown-unknown)	6m 7s
build (stable, thumbv7em-none-eabi)	1m 10s
build (stable, wasm32-unknown-unknown)	48s
doc	18s
minimal-versions	36s
test (1.65.0)	3m 44s
test (stable)	42s
test (nightly)	27s
	21m 0s

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