feat: Enable in-place initialization #244

Job	Run time
miri	46s
tests (1.70.0, serde, false, --skip new_in_place)	30s
tests (stable, true, false)	43s
nostd/thumbv6m-none-eabi/stable	18s
tests (nightly, serde, borsh, zeroize, false)	26s
tests (beta, serde, false)	26s
tests (1.51.0, serde, false, --skip spare_capacity_mut --skip new_in_place)	4m 18s
	7m 27s

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