GPU Hadamard for large N

[barronalex]

Add support for large Hadamard transforms on the GPU.

For $N=2^{24}$ the GPU version is about 50x faster than the CPU:

Timing hadamard_transform ... 2.32494 msec
Timing hadamard_transform ... 123.39948 msec

[angeloskath]

Looks great! As per offline discussion let's move this into the primitive instead.