Make schema dictionaries immutable

[janssenhenning]

One problem I noticed during development of the parsers, is that it is very easy to mutate the schema dictionaries since it is just lists and dicts. So forgetting to copy some entry before doing something with it can have unintended consequences. This is not ideal, when there could be a variety of tools in the future relying on the schema dict

I have some Ideas to implement a subclass of UserDict (the same thing I did in #15 to get a case insensitive key lookup), which can prevent modifications to itself and will immediately raise an error if someone tries.

This is a reminder to myself that I should do something about this at some point