Function env_push 

pub fn env_push(env: &Env, val: Val) -> Env

Push a value onto a borrowed environment, returning a new Env.

The input env is borrowed, so we always need at least one Rc::clone to obtain an owning handle. If anyone else holds a reference to the underlying Vec<Val> (refcount > 1 after the clone), Rc::make_mut will clone the vector for copy-on-write semantics — this is the common case where a parent closure or a sibling kont frame is also looking at the same env.

Prefer env_push_owned when the caller has unique ownership of the Env and is about to discard it (e.g. inside Cont::LetBody after the cont has been popped) — that path avoids the redundant Rc::clone and, when refcount is 1, mutates the vector in place.