Function flatten_neutral_applies 

pub fn flatten_neutral_applies(node: &Node, env: &Env) -> Node

Drop the explicit apply keyword on neutral applications, recursively. (apply f a) whose head is a free constructor-like symbol becomes (f a) so the printed normal form matches the LiNo surface example from issue #50: (succ (succ zero)) rather than the explicit (apply succ (apply succ zero)).