Validate accuracy of standard rules in various domains

[HarrisonGrodin]

We should test our standard rules to confirm accuracy when working in various domains. This was started in #30, but there are still many improvements which should be made, both to the test suite and to the rules themselves.

• The variable substitutions provided by the test suite are very naive, substituting the same test value for all variables. We should test combinations of different variable types.
• We currently only test on integers, which should be changed. We should be testing using various common types (Complex, Matrix, etc.) and updating rule variable images accordingly.
• We currently don't test differentiation, since this is not a functionality that exists in Base, but we should probably test for various values using numerical approximations to validate the rules in DiffRules.jl.