An array with copy-on-write behavior.
CowArray represents either a uniquely owned array or a view of an
'a corresponds to the lifetime of the view variant.
This type is analogous to
CowArray instance is the immutable view variant, then calling a
method for mutating elements in the array will cause it to be converted
into the owned variant (by cloning all the elements) before the
modification is performed.
Array views have all the methods of an array (see
ArcArray, which also provides
copy-on-write behavior but has a reference-counted pointer to the data
instead of either a view or a uniquely owned copy.