Roughly in order of importance:
* Add docstrings.
* Implement indexing of sub-arrays (i.e. a for a 2d-array), perhaps even slicing.
* Implement missing arithmetic operations.
* Implement missing comparisons.
* 0-d arrays should not be sequences. Currently there are problems with using
0-d arrays with PySequence_Fast.
* Merge array_from_arraylike and matrix_from_arraylike.
* Optimize readin_arraylike for elements that are tinyarrays.
* Implement concatenate.