The total number of elements that the array can contain without
/// allocating new storage.
///
/// Every array reserves a specific amount of memory to hold its contents.
/// When you add elements to an array and that array begins to exceed its
/// reserved capacity, the array allocates a larger region of memory and
/// copies its elements into the new storage. The new storage is a multiple
/// of the old storage's size. This exponential growth strategy means that
/// appending an element happens in constant time, averaging the performance
/// of many append operations. Append operations that trigger reallocation
/// have a performance cost, but they occur less and less often as the array
/// grows larger.
///
/// The following example creates an array of integers from an array literal,
/// then appends the elements of another collection. Before appending, the
/// array allocates new storage that is large enough store the resulting
/// elements.