I think there’s a better way to do type checking so I’m going to hold off on it for now and get basic functionality working for now and come back later.

This class exposes two functions. `evalNode` take an AST node and evaluates it down to simplest terms. `eval` takes an array of AST nodes and calls evalNode on each of them. I don’t think we’re quite ready to be producing HTML yet so let’s not get ahead of ourselves.