Detect when package code is out of date

[richfitz]

I don't think that we have build hooks aside from the configure script, but in the meantime, something that could be used to detect whether a model is up-to-date as people do forget to run odin_package

[richfitz]

This will probably use the same machinery as #62; we'll have to hash the model and then can inspect the generated code. Could probably do that on package load, too