Provide dtype='array' fallback

[rreusser]

Of the valid ndarray dtypes, this module does not support 'array'. Does it make sense to provide a fallback that allocates a plain Array if the dtype is array? This would clean up the use of these functions somewhat.

They're trivial but if ndarray-scratch is to implement the canonical zeros, clone, eye, and ones functions, then switching the dtype means having to allocate and fill an array from scratch. If not, then I think ndarray-clone, ndarray-eye, ndarray-ones, and ndarray-zeros need to be split into separate modules that implement this fallback.

(Actually, the main goal is to simplify creation of other modules since there's currently no clean way to allocate an array of the same type as the input.)