restructure and export interface for list of symbolic derivatives

Following up on the discussion in #44, here we export a standardized interface for the list of symbolic derivatives. There are no ``xp`` terms present. It's pretty generic, so I don't think committing to this will place much of a burden on future refactors of Calculus.jl.

The two-argument bessel functions haven't been updated. We could do with a better treatment for functions with multiple arguments, but I don't think it's an issue that needs to be solved for this PR.