ENH: intern some commonly used strings in the multiarray module

Allows improving dictionary lookup performance.
ENH: add a ndarray super-fast path to _array_from_object

When the argument is exactly an ndarray and the rest of the arguments
are their defaults we can skip the very slow
PyArg_ParseTupleAndKeywords.
This improves np.array(ndarray, copy=False) performance by about a
factor three and np.asarray(ndarray) by about a factor of two.
Merge pull request #4902 from juliantaylor/asarray-improve

ENH: add a ndarray super-fast path to _array_from_object