Offer a builder for CUDA arrays and/or array descriptors

[eyalroz]

It would be nicer to let our users build arrays not just with descriptors they provide, but in a proper builder pattern.

Now, we would want a builder to also be usable for building just a descriptor for later use, and so we'll need to choose between a few options:

1. Builder for descriptors, no builder for arrays - you still only construct them using a descriptor.
2. Builder for descriptors, different builder for arrays (so that you don't have to know about descriptors); may share some code under the hood or even have the latter hold the former.
3. Builder for arrays, no builder for descriptors - encouraging users against using descriptors themselves (they can copy the builder around)
4. The descriptor wrapper class will itself be a descriptor for arrays.

I'm tending towards option 4, but not certain yet.