Refactor parsing library and split into separate project

The current state of the parsing library is... not so great. It's very complicated and confusing and results in _way_ too large a file in [ford/sourceform.py]. However, the functionality it implements is very useful and could allow for the development of static analysis tools, linters, etc. As such, we should ideally look towards refactoring this part of the code, simplifying it and reducing dependencies between the parts. The output should also be something more standard than the mess of Python objects it produces now (e.g., JSON, YAML, XML, or some other easily parsed and interpreted serialisation). This would be a major project but would be extremely worthwhile as it would result in the code being simpler, more maintainble, and more widely useable.