Merges two sorted subarrays into a single sorted subarray. This function merges two sorted subarrays of the provided slice into a single sorted subarray. # Arguments `arr` - The slice containing the subarrays to be merged. `left` - The starting index of the first subarray. `mid` - The ending index of the first subarray. `right` - The ending index of the second subarray.
(arr: &mut [T], left: usize, mid: usize, right: usize)