This allows a little bit more of abstraction: we can store other data in the
tree, without having to store it in a specific node (e.g. the variables map,
which is still unused). It also adds a few typehints, and specializes
the return value of eval'ing the AST: previously, it was the one of
evalNode, which wasn't guaranteed to be an AFPData. Now we have this
guarantee. Last but not least, we can now measure runtime metrics for
evalTree, which doesn't recurse.
Bonus: fix a check in the old parser, which used the wrong variable when
reporting outofbounds errors.
Change-Id: Iff806793b1d968e9bb6220f1459f3d0ac587c7da
