Add convienient const helper(hidden behind a feature flag) #240

Job	Run time
miri	47s
nostd/thumbv6m-none-eabi/stable	15s
tests (1.51.0, serde, false, --skip spare_capacity_mut)	5m 11s
tests (1.70.0, serde, false)	28s
tests (stable, true, false)	53s
tests (beta, serde, false)	27s
tests (nightly, serde, borsh, zeroize, false)	28s
	8m 29s

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