add filecoin commitment merkle root codecs

These describe roots & nodes of a merkle tree, not the underlying
data. In the case of CommP and CommD they are binary merkle trees
using sha2-256-trunc2. For CommR they are novel structure merkle
trees using poseidon-bls12_381-a2-fc1.

All nodes of the respective merkle trees could also be described
using this codec if required, all the way to base data. It is
anticipated that the primary use will be restricted to the roots.

Ref: #161
Closes: #161
Closes: #167

table.csv

@@ -432,6 +432,8 @@ skein1024-1016,                 multihash,      0xb3df,
 skein1024-1024,                 multihash,      0xb3e0,
 poseidon-bls12_381-a2-fc1,      multihash,      0xb401,         Poseidon using BLS12-381 and arity of 2 with Filecoin parameters
 poseidon-bls12_381-a2-fc1-sc,   multihash,      0xb402,         Poseidon using BLS12-381 and arity of 2 with Filecoin parameters - high-security variant
+fil-commitment-unsealed,        filecoin,       0xf101,         Filecoin piece or sector data commitment merkle node/root (CommP & CommD)
+fil-commitment-sealed,          filecoin,       0xf102,         Filecoin sector data commitment merkle node/root - sealed and replicated (CommR)
 holochain-adr-v0,               holochain,      0x807124,       Holochain v0 address    + 8 R-S (63 x Base-32)
 holochain-adr-v1,               holochain,      0x817124,       Holochain v1 address    + 8 R-S (63 x Base-32)
 holochain-key-v0,               holochain,      0x947124,       Holochain v0 public key + 8 R-S (63 x Base-32)