replace _spendEncumbrance with _releaseEncumbrance

[scott-silver]

Now that Coburn is back from leave, revisiting the third encumbrance event.

This approach is an alternative to the EncumbranceSpend event from #25. Coburn has pointed out that the logic of _release and _spendEncumbrance are nearly identical.

This patch merges the two functions (renamed to _releaseEncumbrance). This means that when you spend an encumbrance, a Release event is emitted.

If you want to track an address's encumbrance, you can do so by reading Encumber and Release amounts. If you want to track their balance, you can do so by reading Mint, Transfer and Burn amounts.

[coburncoburn]

🧼

[kevincheng96]

Looks good!