More precise includes of `Node` constants (#6617)

 More precise includes of `Node` constants (#6617)

We store constants, e.g., BitVector and Rational, in our node infrastructure. As a result, we were indirectly including some headers in almost all files, e.g., the GMP headers. This commit changes that by forward-declaring the classes for the constants. As a result, we have to include headers like util/rational.h explicitly when we use Rational but it saves about 3 minutes in compile time (CPU time).

The commit changes RoundingMode from an enum to an enum class such that it can be forward declared.