cmd/decode: tool for decoding bc types
cmd/decode: tool for decoding bc types

Add a tool for decoding fedchain/bc types.

Closes chain/chainprv#583.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain