Refactor data accessor callables into data specification objects

[treubig26]

The current Accessor mechanism for handling data format conversions is elegant in its implementation, but is a bit of an abstract concept when describing to new users. Additionally, it is not possible to introspect an accessor to see what the result of using it might be. It is more straight-forward to use an object describing the desired format to which to convert the data, and pass that into a self-describing data type's get/convert/to method.