[DO NOT MERGE] adds PoC model for vault share price valuation

[Tapanito]

Spec: XRPLF/XRPL-Standards#485

High Level Overview of Change

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.9%. Comparing base (7222150) to head (dbf45ae).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           tapanito/lending-fix-amendment   #6434   +/-   ##
==============================================================
  Coverage                            79.9%   79.9%           
==============================================================
  Files                                 878     861   -17     
  Lines                               67970   67941   -29     
  Branches                             7551    7548    -3     
==============================================================
- Hits                                54281   54258   -23     
+ Misses                              13689   13683    -6     

see 54 files with indirect coverage changes

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[ximinez]

Partial review

[ximinez]

In addition to the comments below, I noticed there are a lot of places where you have comments describing the current state of the vault. e.g.

// Vault: assetsTotal=1000, omega=50 (loan outstanding), iota=100, sharesTotal=1000

I'd like to see those have actual BEAST_EXPECT checks. Either added, or replacing the comments, up to your judgement.

Partial review. Note to self: left off at testPaperLossThenActualDefault()

[ximinez]

These changes are good, but I think you abstracted a little bit too much away. A lot of the tests are just verifying that, for example redeemAssets is using the same math as redeem. Doesn't mean the math is right.

What I think would be a good balance is to check the value of all the expected... variables. I'll include a couple of examples below.

[Tapanito]

Thanks @ximinez . Yeah, I think you're right, it was too abstract. I restored some of the concreteness with explicit value assertions.