Add `ArrayVec::spare_capacity_mut` #203

Job	Run time
nostd/thumbv6m-none-eabi/stable	7s
miri	47s
tests (1.51.0, serde, false, --skip spare_capacity_mut)	1m 30s
tests (1.70.0, serde, false)	30s
tests (stable, true, false)	30s
tests (beta, serde, false)	27s
tests (nightly, serde, borsh, zeroize, false)	24s
	4m 15s

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