Hopefully last iteration of the environment-manager logic:
- Manage the sets of undefined macros separately, that is needed in order to treat them correctly.
- A lot of fixes and testing in this area.
- Add more tests.

The immediate effect is that parsing has become slower (It needs some optimization again).
However the environment-management works better now: While editing a document, none of the included headers are reparsed, only the edited document itself, even in complicated cases.