Function call_slice_opt 

pub fn call_slice_opt<const DATA_CAP: usize>(
    contract: [u8; 20],
    calldata: &[u8],
    value: &U,
    gas: u64,
    offset: usize,
) -> Option<(usize, [u8; DATA_CAP])>

Same as the other slice function, though returning Option if error.