Make axis keyword to squeeze() positional

As suggested by @shoyer in #97 (comment)
This makes it possible to predict resulting rank of output array, which is
otherwise undetermined (see discussion in gh-97).

Using squeeze without specifying the axis in library code often results in
unintuitive behaviour. For the common use case of extracting a scalar from a size-1 2-D array,
this gets a little more verbose (e.g. `np.squeeze(np.array([[1]]), axis=(0, 1))`),
but that's probably a price worth paying for the extra clarity.

Also changes specified behaviour for a given axis not having size 1 to raising
a `ValueError`, which is what NumPy does. This wasn't considered before, and the current
description seems simply incorrect.

Finally, this makes `squeeze` the exact inverse of `expand_dims`, which is probably
a good thing.

Author: rgommers

spec/API_specification/manipulation_functions.md

@@ -118,7 +118,7 @@ Rolls array elements along a specified axis. Array elements that roll beyond the
 
     -   an output array having the same data type as `x` and whose elements, relative to `x`, are shifted.
 
-### <a name="squeeze" href="#squeeze">#</a> squeeze(x, /, *, axis=None)
+### <a name="squeeze" href="#squeeze">#</a> squeeze(x, /, axis)
 
 Removes singleton dimensions (axes) from `x`.
 
@@ -128,9 +128,9 @@ Removes singleton dimensions (axes) from `x`.
 
     -   input array.
 
--   **axis**: _Optional\[ Union\[ int, Tuple\[ int, ... ] ] ]_ 
+-   **axis**: _Union\[ None, int, Tuple\[ int, ... ] ]_
 
-    -   axis (or axes) to squeeze. If provided, only the specified axes must be squeezed. If `axis` is `None`, all singleton dimensions (axes) must be removed. If a specified axis has a size greater than one, the specified axis must be left unchanged. Default: `None`.
+    -   axis (or axes) to squeeze. If `axis` is `None`, all singleton dimensions (axes) must be removed. If a specified axis has a size greater than one, the specified axis must be left unchanged.
 
 #### Returns