Improvements to header tests

[opalmer]

Right now the header tests, as implemented in #14, are kind of basic. Ideally we need to be sure of a couple of things:

• Anything being defined in a header file must end up on the built library object. The tests already cover this but using regular expressions or string manipulation to discover the exported entries is flaky. It would be better if we parsed the syntax instead in the tests.
• Anything that used to be defined in a header file remains defined (ie a definition can't be removed by accident). Although this library still under development it's good policy to get into this habit now so compatibility is not broken by accident. A small tool to parse/dump a reference object for use in the tests would probably be sufficient.