Add support for diffing expressions with indexed vars in differentiate2c

[JCGoran]

Note that the independent var we are diffing against should not be an indexed var (yet), but every other var is fair game.

[1uc]

one could still isolate it to make sure there's one notion of how to convert random stuff to symbols.

[JCGoran]

one could still isolate it to make sure there's one notion of how to convert random stuff to symbols.

In principle yes, but I feel that it would undoubtedly involve a bunch of heuristics (much like what SymPy is actually doing under the hood anyway). The least error-prone idea is to just put sympy_vars = {str(var) : var for var in vars}, and require the user passes an iterable of sympy objects (like [sp.Function('f'), sp.Symbol('x')]), and raise an error if they pass anything else.