* Modify `parse_expression` to take a controlling (parent) precedence.
* `parse_expression` folds left over sequences of sub-expressions with decreasing operator precedence (accumulates in `lhs`).
* Use recursion rather than accumulator for left fold in `parse_binop` to simplify code logic.
* Extend parser unit test to cover more complicated, multi-level expression.
* Remove (now) redundant parenthesis from derivative check block in kinetic rewriter test.

Fixes #94