Better handling of geom and stat params

[hadley]

Would be nice to have consistent way of discovering parameters so that layer can warn early if you have parameters that don't exist in the geom/stat.

Probably the easiest way would be to provide a character vector that lists the names of the parameters. This will require some manual book-keeping, but you should notice quickly during testing if you've forgotten to add a param.

Then should be able to eliminate use of ... in many of the individual methods. (Will still need it in the default compute_panel() because it passes onto compute_group()