FP converter: convert: Use std::vector as instead of std::stack. (#3563)

 FP converter: convert: Use std::vector as instead of std::stack. (#3563)

The word-blasting function `FpConverter::convert` used a `std::stack` with `deque` as an underlying data structure (default) for node traversal. Previous experiments suggested that using `std::stack<T, std::deque<T>>` performs worse than using `std::vector<T>`, and we decided, as a guideline, to always use `std::vector` for stacks: https://github.com/CVC4/CVC4/wiki/Code-Style-Guidelines#stack.

This PR refactors `FpConverter::convert` to use `std::vector`. Runs on all incremental and non-incremental FP logics in SMT-LIB showed a slight improvement over the previous (`std::stack<T, std::deque<T>>`) implementation, and an even greater (albeit still slight) improvement over using `std::stack<T, std::vector<T>>`.