Make TypeDescriptor more easily extended

[johnynek]

https://github.com/twitter/scalding/blob/398545991c63019714d80ac22de6494adfd798f2/scalding-core/src/main/scala/com/twitter/scalding/macros/impl/TypeDescriptorProviderImpl.scala

If users override an implicit TypeDescriptor, use that if possible.

[johnynek]

also it should be composable. I think it can have a product or zip method to join two together, and similarly, if you give me a bijection, I can produce a new TypeDescriptor from the old (it is an invariant functor and cartesian).

Using the cartesian instance, the macro should be able to compose from there.