support for nested arrays reassignment and read

[shubhiscoding]

let x = [10, [30, 4], 5];
print x[0];
print x[1][0];

support for x[1][0]

also Need to switch to Rc<RefCell<Vec>> to gain O(1) read and write

[shubhiscoding]

Nested read fixed by #6
Nested write fixed and refactor to Rc<RefCell<Vec>> by #7