Infer a type of captured free variable in function body

[coord-e]

1. Implement Value::free_variables
2. Remove check_captures and replace its uses with that above
3. Use Value::free_variables in TypeInfer::transform_with_env and rollback new_env changes to env

[coord-e]

test code:

let zero = (f, x) -> x;
let succ = n -> (f, x) -> f(n(f, x));
let plus = (m, n) -> m(succ, n);  // !
let toint = f -> f(x -> x + 1, 0);
let four = plus(succ(zero), succ(succ(succ(zero))));
toint(four)  // 4

[coord-e]

3. doesn't work!