Using this, the vPSBT packet now contains the information required to
have a distinct asset version for a given output.

bip-tap-psbt: define value for asset version

Use even/odd TLV numbers everywhere

To avoid confusing it with the "next" asset_script_key and asset_id, we
rename the fields slightly.

This let the verifier check that the `asset_group_key` is derived
correctly from the internal key, and that the signature is valid.

…tion

In this commit, we split the Universe trees into transfer vs issuance.
The leaf sum value for issuance is the number of units, while for
transfer just `1` (accumulator).

For Multiverse trees, we make a similar distinction. The sum value for
an issuance multiverse is just 1, so it tracks the total amount of
assets committed to. For transfer multiverses, the value here is the
same as the root of a transfer universe, this ends up summing to the
total number of transfer committed to.

bip-tap-universe: split universe trees, use new sum values

The field `asset_tree_root` is not 40 bytes long, but 32.

This is both consistent with the implementation, and with the definition
of the field further down in this bip.

Definition:

> the asset tree is calculated as either:
> * <code>asset_tree_root = sha256(asset_id || left_hash || right_hash
> || sum_value)</code>
> or
> * <code>asset_tree_root = sha256(asset_group_key || left_hash ||
> right_hash || sum_value)</code>

Implementation:

```
leafParts := [][]byte{
  {byte(c.Version)}, TaprootAssetsMarker[:], rootHash[:],
  rootSum[:],
}
```

The bip now reflects this.

bip-tap: correct `asset_tree_root` usage