decode cid

[SimonLab]

ref #13 #15

• Check for v0 format, ie start with Qm and length is 46 (not sure why about the length)

[nelsonic]

@SimonLab we don't need to decode v0.
Apologies for not making that clear in #11
We only need to do what is listed in #11.
We need to generate a few test CIDs using the JS version
and then test those in our Elixir version, that's it.
No decoding required at this stage.

If you feel this is "almost done" that's fine we can merge it.
But if you feel there is still 4+ Hours of work to do, then let's "shelve" it
and focus on creating V1 CIDs compatible with JS.

[SimonLab]

closing as the decode function is not necessary. The PR #21 is also more up to date