bip-taro+assets: add explicit input split commitment to asset TLV

[Roasbeef]

Related to: #5

Today we have a split commitment for the set of inputs. This is useful for the async send where we want to create something on chain then transmit the other relevant pieces like the split commitment proof/root to the receiver(s).

I think it makes sense to also add a similar commitment but for the set of inputs. During VM validation, we already create such an input commitment here, which we later use to verify that no asset inflation happened during the transfer.

The advantage of pulling up the computation of this commitment a few layers is the following:

1. Easier to extract info at a glance from an asset TLV leaf (total consumed, if a full spit was needed, etc)
2. Similar to file: use append only merkle-tree or SMT in the file format to allow for probabilistic verification?  #5 lets ppl do probabilistic validation of inputs/merges if they want do