Implement as_flattened{_mut}() for Array<Array<>>

[jmwample]

Implementation of as_flattened and as_flattened_mut for nested Array types to make it easier to operate on nested Arrays. Inspiration drawn from the as_flattened{_mut} implementations on [[T; N]] (i.e. for the slice primitive).

Note:

• Array<[T; N], U> already implements these functions as it is transparent to &[[T; N]].
• [Array<T, U>] does not implement these functions and we can't implement them this way without a new local trait. Doesn't really seem necessary though.

[tarcieri]

NOTE: the SAFETY comments could probably include some more information regarding the Array type always being a newtype for a core array, similar to the existing SAFETY comments, but this is fine for now