Removing hardcoded filename in ast.cpp

This patch replaces the hardcoded filename in error messages from `ast.cpp` with the more maintainable form `__FILE__`.

An odd thing I stumbled on while reading `ast.cpp` is that `getReverseOpName()` defines the reverse of `AST_TYPE::NotEq` to be itself. The same happens with `AST_TYPE::Eq`. I tried to fix it, but while doing so, the tests `comparisons.py` and `tuples.py` failed.

I also made some slight modifications in `INSTALLING.md`, and added a note that `zsh` is now needed.