Standardise collection clearing with Array.Clear()

Arrays currently remove every element with RemoveAll(), while HashSet.Clear() and Dictionary.Clear() clear their collections. This request adds Array.Clear() as the preferred API for removing every element from an array, so all built-in data collections share the same clearing idiom.

Array.Clear() is now the current clearing API. RemoveAll() is retained only for source compatibility: it is deprecated, and existing programs using it keep compiling and running while the compiler emits the normal deprecated-API warning recommending Clear().