Move node visitor class from smt_util/ to expr/ (#4110)

Move node visitor class from smt_util/ to expr/ (#4110)

Done by:

Running rg 'smt_util/node_visitor' -l | xargs sed -i 's/smt_util\/node_visitor/expr\/node_visitor/' in src to change the #includes
Moving the file
Changing src/expr/CMakeLists.txt and src/CMakeLists.txt
clang-format, omitting node_visitor.h.
In reference to discussion, here.