This PR makes a slight change to the existing code in uucore/features/ringbuffer.rs.

Currently, the RingBuffer<T> struct's new() function creates the ringbuffer with an empty VecDeque<T> that does not reserve a certain capacity for elements. Internally, VecDeque<T> resizes and copies the element of its buffer to a new double sized buffer when it's at capacity and push_back() call is invoked. For a fixed-size ringbuffer, instead of paying the cost of resizing the buffer at every power of 2, we can pay the cost of directly allocating exactly the memory we want from the requested size in the new() function upfront to avoid resizing. It also removes the need for a size field in the RingBuffer<T> struct because VecDeque<T> has a capacity() method that tells you how many items the buffer can contain without reallocating.