Add convienient const helper(hidden behind a feature flag) #239

Job	Run time
nostd/thumbv6m-none-eabi/stable	17s
tests (1.70.0, serde, false)	29s
miri	47s
tests (beta, serde, false)	27s
tests (nightly, serde, borsh, zeroize, false)	27s
tests (1.51.0, serde, false, --skip spare_capacity_mut)	4m 58s
tests (stable, true, false)	36s
	8m 1s

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