Update health precision from UFix64 to UInt256 with 18 decimals

[sisyphusSmiling]

Description

• Increases the decimal precision of the health metric from 8 to 18 decimals, necessarily updating the type from UFix64 to UInt256

[codecov]

Codecov Report

Attention: Patch coverage is 88.63636% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cadence/contracts/TidalProtocolUtils.cdc	62.50%	3 Missing ⚠️
cadence/contracts/TidalProtocol.cdc	94.44%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[sisyphusSmiling]

Default variance of 0.00000001 (as int) which is necessary in instances where withdrawal amounts at target health are truncated - e.g. 1,000 FLOW == 800 effective collateral / 1.3 target health == 615.384615384615384615 available which when converted to UFix64 withdrawal amount leave 0.000000004615384615 available in excess of target health.

[sisyphusSmiling]

@nialexsan added some test cases to cover fundsRequiredForTargetHealth() as well as a fix in b4ee545. There was an oversight in #25 that assigned uintDepositPrice using the price for withdrawType.

Still, I'm not sure the expected & actual values are being computed correctly in fundsRequiredForTargetHealth(). @kgrgpg could you double check my work in the way the expected values are being computed and the logs to ensure the method is operating as expected?

Update: I think I fixed the issue in 3627bea. I believe the borrow factor had to be accounted for in the final calculation.