feat: change native_account::remove_asset to return the remaining asset

[PhilippGackstatter]

Changes

Refactors native_account::remove_asset to return the asset value remaining in the vault after the operation in order to make the return value more useful.

• For that, the asset_vault::remove_asset API was changed to return the remaining value.
• Since faucet::burn also made use of that API, it had to be updated as well. The strategy there is to remove the return value since it is not meaningful (described in Avoid returning outputs identical to inputs in kernel procedures #2523).
• To keep the API logic in MASM and Rust the same, AssetVault::remove_asset was changed to return not the removed asset but the remaining asset, represented as Option<Asset>, where:
  • None is returned for non-fungible assets since nothing remains in the vault
  • and the remaining asset for fungible assets.
    • We could technically return FungibleAsset at this point in time, but this will likely become Asset in the future anyway.
• When adding or removing fungible assets from the vault, there was a previously useful optimization to check if the remaining fungible asset representation's was [amount = 0, 0, faucet_id_suffix, faucet_id_prefix] and if so, insert an empty word to avoid storing zero amounts to keep the merkle tree sparse. Since the asset refactor, this is no longer necessary and so this optimization was removed.

Migration

• miden::protocol::native_account::remove_asset returns the asset value remaining in the vault instead of the removed value
• AssetVault::remove_asset returns the asset value remaining in the vault Option<Asset> rather than the removed value Asset.
• miden::protocol::faucet::burn no longer returns the burnt asset value.

closes #2524