Added
- new options to cvc5 solver config
- `default_solver_config` constant so you don't need to initialize
  it each time
- `print_assertions` method that converts smt lang to something more
  readable
- tests for solver class

- Changed `^` opeartor in FFITerm since it's not compatible with integers

- Changed `!=` operator so it now uses boolean not instead of `== false`

- Updated `std::string` method, so it now uses my new `stringify_term`
  function

- Added `!` operator to symbolic Bool class