define hashable content for Subs, Lambda, Integral, Sum and Product

The new definition allows expressions that would give identical
results (indepenedent of bound variables) to compare the same.

Issues 2440, 3775
make Lambda variables Symbol, not Dummy
test and sympify Lambda input
minor ode cleanup

the global declaration of y was moved to the top of the function
and update the inline comments in _handle_Integral which no longer
apply

retlist processing was handled as a list comprehension
consolidate sum-like methods; disambiguate 1 and 2 arg limits

Product and Sum now use the consolidated _as_dummy method.
remove hashable content changes to Sum, Product and Integral
Merge pull request #2066 from smichr/fold

defining hashable content for objects with bound symbols