Function cookie_factory::cond

pub fn cond<F, I>(condition: bool, f: F) -> impl SerializeFn<I> where
    F: SerializeFn<I>, 

applies a serializer if the condition is true

use cookie_factory::{length, cond, string};

let mut buf = [0u8; 100];

let len = {
  let (len, _) = length(cond(true, string("abcd")))(&mut buf[..]).unwrap();
  len
};

assert_eq!(len, 4usize);
assert_eq!(&buf[..4], &b"abcd"[..]);