Support into iterator

[hackaugusto]

Removes the Sized bound, it is no longer necessary, since #239 .
The change above allows for impl<T: Serializable> Serializable for &T.
The change above allows to change write_many to be generic and receive a IntoIterator without breaking the serialization for [T; C]

The removal of the Sized bound also makes the trait object save, and allows for type erasure.

[irakliyk]

Thank you! Looks good!

btw, one other interesting thing we can improve on serialization front: we could now implement Serializable and Deserializable for Vec, String, and maybe &[] by first serializing the number of elements using as usize and then serializing the actual elements.

For example, for Vec it would look something like this:

impl <T: Serializable> Serializable for Vec<T> {
    fn write_into<W: ByteWriter>(&self, target: &mut W) {
        target.write_usize(self.len());
        target.write_many(self);
    }
}

[hackaugusto]

@irakliyk yes! The motivation for this was the BTreeMap serialization. This also allows that to be serialized with no copying (whereas now one has to copy the map to a vec and then serialize that)