getitem/setitem putmask hooks

[tdimitri]

boolean and fancy index masking related routines can be sped up and are used very often.
Be nice if we could hook these routines.

[mattip]

We may have to hack the tp_getitem, tp_setitem to do this.

[teoliphant]

Yes, to replace "mask" and fancy-indexing, generically, we would likely need to replace the Python-level functions. There might be a way to speed things up a bit by replacing functions on the "PyArray_ArrFuncs *f" member of the PyArray_Descr ("dtype") object. There are functions there that also need to be swapped out to speed up sorting, and such. See lines near 443 to 553 of ndarraytypes.h (though I may have an old version of the NumPy code checked-out).