added type for differentiate in chain_rules.jl

[harris-mit]

1. We should make sure that differentiate is being called on a polynomial p. (It is possible there are other meanings of differentiate for other types in the code.)
2. The function "pullback" is very generic, so I added some clarity.

[harris-mit]

Above I mean the function name "pullback" is generic, so I made the name more descriptive.