FromBytes::insert_vec_from_bytes

[kupiakos]

FromBytes::insert_vec_from_bytes is like FromZeros::insert_vec_zeroed, but accepts bytes: &[u8] to copy instead of an additional: usize zeroed elements to insert. It's an optimization of:

if bytes.len().checked_rem(size_of::<T>()).unwrap_or(bytes.len()) != 0 {
    return Err(/* bytes.len() is not a multiple of the size of T */);
}
let additional = bytes.len().checked_div(size_of::<T>()).unwrap_or(0);
T::insert_vec_zeroed(&mut values, position, additional)?;
// The panic should be optimized away due to the above rem check.
values[position..position + additional].as_mut_bytes().copy_from_slice(bytes);

• It should not have a zeroing memory step.
• The capacity * size_of::<T>() of the returned Vec<T> should match the size of the input to ensure into_boxed_slice doesn't need to realloc.
• bytes does not need to be aligned, same as read_from_bytes.
• Note that if T is a ZST, then the only valid input for bytes is &[], which is a no-op.
• extend_vec_from_bytes is also included in this design, with no position parameter.
• FromBytes::new_vec_from_bytes(bytes: &[u8]) copies into a new Vec<Self>
• Open question: should it panic when position > v.len() given FromZeros::insert_vec_zeroed both panics and returns Result #2242?
• Open question: what's the best Err return type?