Add benchmarks that compare two ways of checking for
`multihash.IDENTITY` code:
1. `Cid.Prefix().MhType`
2. Decode of `Cid.Hash()`
This benchmark illustrates that using Cid.Prefix is more efficient than
`multihash.Decode`. Users wishing to perform such a check should use
`Cid.Prefix`.
Consider that `Cid.Prefix` is already efficient enough and introducing a
dedicated API for performing this check will likely result in small
gains.
Relates to #133
