Add a simple demo of pymbolic.

test/simple.py

+from pymbolic import parse, var
+from pymbolic.mapper.dependency import DependencyMapper
+
+x = var("x")
+y = var("y")
+
+expr2 = 3*x+5-y
+expr = parse("3*x+5-y")
+
+print expr
+print expr2
+
+dm = DependencyMapper()
+print dm(expr)