assets: decide on if genesis asset details serialization should be explicit or implicit

Nobody has claimed this yet.

Our current serialization actually writes all the genesis asset details (genesis point, name, tag, etc) out to the TLV serialization. In contrast, the BIP only writes the "core" asset information, and sort of assumes that the genesis information "is known".

The current state of https://github.com/lightninglabs/taro/pull/49 puts us somewhere in the middle: the genesis information now holds the type, and we continue to write the type out in the main asset body as well.

In short, we should decide on one or the other. I'm somewhat in favor of just going all the way and making things _fully_ explicit. This would mean all the base genesis information is transmitted along side the core asset, and also will be a part of the asset TLV leaf. This is sort of already there implicitly, since we have an explicit field for the `asset_id`.

Review the current TLV serialization of genesis asset details and the changes introduced by PR #49, then compare them with the BIP's core-only model. The work is complete when one explicit or implicit representation is decided and applied consistently to the main asset body and asset TLV leaf.

Written by the indexing model from the issue text.