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)