Rework symbol table

[famoser]

The current symbol table does not collect all symbols. Notably, new declarations are not supported everywhere. Further, the symbol table treats lib statements as global context (hence not one-after-the-other, but all on the same level). This causes some invalid navigations (e.g. navigating to quantifier of different clause). Lastly, the behavior of macros is not clearly defined, and should be thought through.

This PR approaches this in the following way:

• Implement statements which define a variable directly (rather than in their parent rule)
• Correctly track scope of the lib (and any other statement)
• Define and implement clear macro behaviour