SymEngine doesn't work after rscale changes

When I reviewed the rscale patch I guess I didn't actually test that the code worked with SymEngine (I thought I did but I guess the unit tests I ran did not exercise everything).

• matrix / scalar doesn't work. matrix * scalar**-1 works instead (This seems like it ought to be fixed in SymEngine).
• deep is not a kwarg to .expand() in SymEngine
• subs() takes a dict as argument, not a list