convert to iterative expression parsing

Switch the expression parsing from [Pratt Parsing](https://matklad.github.io/2020/04/13/simple-but-powerful-pratt-parsing) to [Shunting Yard's Algorithm](https://matklad.github.io/2020/04/15/from-pratt-to-dijkstra.html) as it avoids any stack overflow issues. Though, I think Rust will support tail call optimization soon.