[WIP] Initial SymEngine integration.

sumpy/assignment_collection.py

sumpy/codegen.py

sumpy/cse.py

sumpy/e2e.py

sumpy/e2p.py

sumpy/expansion/__init__.py

sumpy/expansion/local.py

sumpy/expansion/multipole.py

sumpy/kernel.py

sumpy/p2e.py

sumpy/p2p.py

sumpy/qbx.py

sumpy/symbolic.py

sumpy/tools.py

test/test_codegen.py

test/test_cse.py