Cleanup in transcendental solver, add ApproximationBounds struct. (#5945)

Cleanup in transcendental solver, add ApproximationBounds struct. (#5945)

This PR merges some cleanup in the transcendental solver from proof-new.
It adds a new struct ApproximationBounds that replaces an opaque std::vector and does some general refactoring in the TaylorGenerator class, removing dead code and using fixed-width integers.