Properly handle CIDs

[Gozala]

Implementations should decode Links as CIDs rather than arbitrary bytes. More specifically following enum representation:

rust-ipld/core/src/lib.rs

Line 13 in a508ee6

Link(Vec<u8>),

Should change to:

pub enum Ipld {
 Null,
 Bool(bool),
 Integer(i128),
 Float(f64),
 String(String),
 Bytes(Vec<u8>),
 List(Vec<Ipld>),
 Map(BTreeMap<String, Ipld>),
 Link(CID),
}

Where CID is cid::CID.

@vmx does that sound like a reasonable plan ?

[vmx]

Yes that sounds right.