Form Helper

This is a WIP, please feel free to comment or suggest changes. An end goal for
me to is have something like
[SimpleForm](https://github.com/plataformatec/simple_form/) so hopefully
@josevalim will have some thoughts.

This will also rely heavily on the `Resource` protocol discussed in #320.
I would also remove the Access here. We only seem to use it for `get_in/2` below and `get_in/2` does not seem to be the best option there because we actually want to assert the resource field exists in the builder since if it doesn't, someone passed an invalid builder.