fix(#439): updates trading function bound case v1.4.0-beta

[Alexangelj]

Changes

Updates the NormalStrategyLib.tradingFunction to explicitly handle the boundary case.

• Adds four new custom errors that are used when one reserve is at the bound but the other reserve is not at the opposite bound.
• Adds two branches of code in trading function:
  • 1. If x >= WAD || y <= 0, then checks if the other reserve is at the opposite bound, if not it reverts, else it returns σ√τ
  • 2. If x <= 0 || y >= WAD, then checks if other reserve is at the opposite bound, if not it reverts, else it returns σ√τ
  • 3. Else, computes the trading function regularly

There needs to be some updates to the test suite to test these boundary cases better. Right now, there are fuzz tests that fuzz a broad space of parameters and reserves, but these should be more concentrated on fuzzing the reserve changes against a few specific parameter profiles.