fix calculate_implied_rate to exclude the backpaid interest from the cost basis

[MazyGio]

Resolved Issues

#119 Implied rate (Short ROI) is negative in some cases where fixed > variable

Description

This excludes the short's backpaid interest from implied_rate calc

Review Checklists

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• Testing
  • Are there new or updated unit or integration tests?
  • Do the tests cover the happy paths?
  • Do the tests cover the unhappy paths?
  • Are there an adequate number of fuzz tests to ensure that we are
    covering the full input space?
  • If matching Solidity behavior, are there differential fuzz tests that
    ensure that Rust matches Solidity?

[dpaiton]

This lgtm although I think it would be a good idea to put a test together that demonstrates how it works as a follow-up.