Flutterby™! : Yay got my first basic numeric

Yay, got my first basic numeric expression parser up with Rust, now trying to add the features my Objective-C version has, better debugging, figuring out how I tie code to the nodes, but I can express a BNF-like language directly in code, inspect the parse tree at run-time for things like autocomplete, and it's teaching me how to think in Rust.

Which seems to involve a lot of .clone() calls, I'm gonna have to look at the output machine code to see what it's really doing.