Function build_plan_digest 

pub fn build_plan_digest<'output, 'plan, 'request, H: FingerprintHasher>(
    plan: PlanConfirmation<'plan, 'request>,
    scratch: &mut [u8],
    output: &'output mut [u8],
    hasher: &H,
) -> Result<PlanFingerprintDigest<'output, 'plan, 'request>, PlanFingerprintBuildError<H::Error>>

Builds a caller-selected collision-resistant digest and clears scratch.