Add new Destroy output

[xgreenx]

The current implementation: If a transaction has a non-zero balance for the asset at the end of the transaction execution and doesn't have a corresponding Change output, we destroy(or burn) tokens. This mechanism allows the destruction of tokens without the use of the contract.

An implicit mechanism like this sometimes is hard to understand, and if the user accidentally misses the Change output, the token will burn.

We want to make this process more explicit by introducing the Destroy output.

When we have this new output, we can require transactions to have either Change output or Destroy for each of the assets from the inputs.