Add calculate_shares_delta_given_bonds_delta_derivative

[sentilesdal]

Resolved Issues

This is a part of adding functions needed to test remove_liquidity

Description

• Adds the lower level function calculate_shares_delta_given_bonds_delta_derivative to lp.rs
• Adds a new version of calculate_present_value_safe that doesn't panic. Future work will remove the previous version.
• Adds a helper function to calculate idle reserves
• Tests calculate_shares_delta_given_bonds_delta_derivative.

I ran diagnostics to make sure that we were actually testing the function, the test errors on the present value calculation about half the time, errors on the actual function 1/4 time and succeeds about a 1/4 of the time.

Review Checklists

Please check each item before approving the pull request. While going
through the checklist, it is recommended to leave comments on items that are
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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?

[jalextowle]

The impl looks right to me. A few style nits that should be addressed before merging, but otherwise LGTM

[dpaiton]

approved pending replies to our comments