Adds Jet, modifies RingTest

Hello spirer's

with apologies for the long delay after my announcement, here is my implementation of Jet's for automatic differentiation, based on Complex. 

Compiles, tests pass and it seems to do what is supposed to do on its own. However, I noticed that Complex(-Instances,Algebra,etc) has undergone some changes since I started months, and this may explain why I am having some difficulties integrating with Numeric. However, I was able to add Jet to RingTest with minimal modifications.

Your comments are very welcome.

Thanks
Franco