Make sure that all cdata references for variables is tracked. This is
done by attaching finalizers to Expression and Term objects. Some
effort is made to avoid creating tracked temporary objects (esp Terms)
now.
LuaJIT FFI is not inherently memory safe and there is no way
to completely guard against the caller doing something that
will trample over memory, but we can get pretty close. Biggest
issue is that an empty table will stand-in for a ref struct with
a null ref. So check for that in all the calls. In the calls that
raise errors we now have a specific error for it. In the other
functions the "nil" object is handled quietly but without a nullptr
dereference and hopefully no UB.
