Change Cid to depend on Multihash for storage

[rklaehn]

There is an optimization in multiformats/rust-multihash that stores the hash inline if it is is small. For rust-cid to benefit from this it would have to use a Multihash as storage in Cid instead of a Vec...

[rklaehn]

See multiformats/rust-multihash#47

[vmx]

Fixed with #32.

@rklaehn It would be cool if you could mention related issues in the commit message, so that they get closed automatically on merge.