perf(serde): avoid some allocations when deserializing #450

Job	Run time
Multiple toolchain tasks (beta)	31s
fuzz (--fuzz-dir=keyvalues-serde/fuzz loosely_typed)	2m 50s
fuzz (--fuzz-dir=keyvalues-parser/fuzz parse)	2m 42s
Multiple toolchain tasks (stable)	32s
fuzz (--fuzz-dir=keyvalues-serde/fuzz serde)	2m 49s
	9m 24s

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