This introduces a TermManager object, which will, in the future, be the
sole object responsible for handling/managing terms and sorts. For now,
all corresponding functions in `cvc5::Solver` are marked as deprecated,
as is constructor `cvc5::Solver::Solver()`, since in the future a solver
instance must be constructed from a term manager instance. Currently, we
maintain a static thread_local term manager instance to not break the
API and continue providing constructor `cvc5::Solver::Solver()`.
Note that this already converts all C++ unit tests to use the
TermManager except for a single test `getStatistics()` in
`test/unit/api/cpp/solver_black.cpp`. Statistics handling is currently
still maintained on the solver level. The statistics we maintain,
however, concern terms only and will eventually be refactored to be
tracked in the NodeManager.
Further note that the Java and Python APIs will be refactored in
separate PRs.
Organizing the PR a bit:
we hook up the subtheories to TheoryFf
we expose FF-related things via the C++/Pytohn API and SMT-LIB2 interface.
we add a bunch of tests against these interfaces.
