The array creation functions have the most to gain:

1. np.asarray is 4 times faster and commonly used.
2. Other functions are wrapped using __array_function__ in Python
   making it more difficult

This commit (unfortunatly) has to do a few things:

* Modify __array_function__ C-side dispatching to accomodate
  the fastcall argument convention.
* Move asarray, etc. to C after removing all "fast paths" from
  np.array (simplifying the code)
* Fixup imports, since asarray was imported directly in a few places
* Replace some places where `np.array` was probably used for speed
  instead of np.asarray or similar. (or by accident in 1 or 2 places)

* Fixed  keyword bug

* Added test case

* Reverted to original notation

* Added tests for var and std

Closes gh-18552

This removes a 20%-30% overhead, and thus the largest chunk of
slowdown incurred by adding the `where` argument. Most other places
have fast-paths for `where=True`, this one also should have it.

The additional argument does slow down the function versions a bit
more than this, but that is to be expected probably (it has to
build a new argument dict, at some point we might want to move this
to C, but that seems worth much more with FASTCALL logic).

Harmonize the signature of np.mean, np.var np.std, np.any, np.all,
and their respective nd.array methods with np.sum by adding a where
argument, see gh-15818.

Catch IndexError in _count_reduce_items used in np.mean and np.var for
illegal axis and reraise as AxisError, see gh-15817.

Removes unnecessary code introduced in #15696.

Non-native byte orders were explicitly added to the fast-path
check in _var for complex numbers. However, the non-native
path is unreachable due to coercion in upstream ufuncs.

var currently has a conditional that results in conjugate being
called for the variance calculation of complex inputs.

This leg of the computation is slow. This PR avoids this
computational leg for complex inputs via a type check.

Closes #15684

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.