fn expand_prehash(prehash: Vec<u8>, length: usize) -> Vec<u8> ⓘExpand description
A helper function that expand a prehash to a specified length if the prehash is shorter than the specified length.
If the length of prehash is less than length, return a prehash with length length
constructed by left-padding prehash with zeros. Otherwire, return the unmodified prehash.