Allow `serde` without `alloc` #2599

Job	Run time
build (stable, wasm32-unknown-unknown)	37s
build (1.85.0, wasm32-unknown-unknown)	30s
build (1.85.0, thumbv7em-none-eabi)	37s
test (stable)	33s
test (1.85.0)	37s
build (stable, thumbv7em-none-eabi)	34s
minimal-versions	0s
	3m 28s

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