try and remove cyclic deps in symbolic module

examples/advection/var-velocity.py

examples/dagrt-fusion.py

examples/wave/var-propagation-speed.py

grudge/__init__.py

grudge/models/advection.py

grudge/models/em.py

grudge/models/wave.py

grudge/sym.py

grudge/symbolic/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

test/test_grudge.py