feat: Enable in-place initialization #242

Job	Run time
miri	44s
nostd/thumbv6m-none-eabi/stable	17s
tests (stable, true, false)	26s
tests (nightly, serde, borsh, zeroize, false)	25s
tests (beta, serde, false)	28s
tests (1.51.0, serde, false, --skip spare_capacity_mut)	33s
tests (1.70.0, serde, false)	21s
	3m 14s

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