Add tombstone handling to `asset.New`

[jharveyb]

https://github.com/lightninglabs/taro-private/pull/30#discussion_r1186565730

-Update asset.New to reject 0-value assets that do not use the NUMS key
-Error on assets with the NUMS key and non-zero amounts

This may have to change once we sort out the standard for burning.

[jharveyb]

Still relevant since the finalized burn spec does not use the NUMS key directly.