parse_skipped can't parse zeros

[orlp]

parse_skipped strips all leading zeros, but this will cause atoi_simd::parse_skipped(b"0") to return Err(Empty).

Some test cases that should all parse to zero:

assert_eq!(parse_skipped::<i8>(b"0"), Ok(0));
assert_eq!(parse_skipped::<i8>(b"-0"), Ok(0));
assert_eq!(parse_skipped::<i8>(b"-0000000000000000000000000000"), Ok(0));
assert_eq!(parse_skipped::<i8>(b"+0"), Ok(0));
assert_eq!(parse_skipped::<i8>(b"+0000000000000000000000000000"), Ok(0));
assert_eq!(parse_skipped::<u8>(b"0"), Ok(0));
assert_eq!(parse_skipped::<u8>(b"+0"), Ok(0));
assert_eq!(parse_skipped::<u8>(b"+0000000000000000000000000000"), Ok(0));

and repeated for i16, i32, i64, i128 and u16, u32, u64, u128.