Upgrade to typed memory views

[matt-chan]

See http://cython.readthedocs.io/en/latest/src/userguide/memoryviews.html#comparison-to-the-old-buffer-support

It speeds things up significantly, and also gives us a better chance of having some sort of parallelism.

I think the change can be made mostly through a few clever 'sed's.

[tovrstra]

There is a catch, but luckily not too relevant for us: Cython memoryviews do not support read-only buffers. cython/cython#1605. We just need to keep it in mind.