ioreg syntax extension

I'll need to write something more coherent here but here is the continuation of the ioreg work originally discussed in #90. I've restructured and cleaned up the builder code substantially and getter and setter objects have been implemented. Note that this currently requires some [compiler bits](https://github.com/rust-lang/rust/pull/15732) that have yet to be merged to compile. You can find rustdoc output for the code generated by a simple example [here](http://mw0.mooo.com/~ben/doc/ioreg_test/). More on this later when it's not as late.

I would apply the proper label to indicate this is work in progress but it seems I don't have permission to do so.

## Remaining work

 * Examine options for reducing documentation clutter
I'll admit that I'm not a huge fan of this aspect of the style guidelines as it makes it much less clear where the function's signature ends and the body begins. That being said, I've made the changes.