Significantly improved the parser.

- Thought over and reconfigured precedences. These are now also synchronized
  with precedences in the line printer.
- The subscript operator ("_") has been added to the grammar similarly to
  exponentiation ("^"). This resolved some issues with the use of operators in
  integral bounds.
- Unary negation now has a higher precedence, but the parser moves negations up
  in the tree as far as possible (within the bounds of correctness and without
  causing cycles during reduction).
- All unit tests have been updated using the new syntax.
- graph_drawing has been updated to include the new syntax of the line printer.