OUSD Better Burn Mint

[DanielVF]

Changing over burning and minting to work from a balance centric point of view.

On burns this provides:

• Can burn any amount
• Can always burn account balance

While this passes current tests, and should be perfectly accurate for a hundred years worth of yield, it would be good to characterize worst case rounding results when resolution is insufficient.

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Contract change checklist:

• Code reviewed by 2 reviewers.
• Copy & paste code review security checklist below this checklist.
• Unit tests pass
• Slither tests pass with no warning
• Echidna tests pass if PR includes changes to OUSD contract (not automated, run manually on local)