c.parallel should reuse CUB policies directly

[griwes]

Currently, c.parallel contains copies of some of the tuning parameters of CUB algorithms, and uses those copies. This is unsustainable; for simpler algorithms it doesn't take much code, but for, say, RadixSort, this would be a massive block of logic. Additionally, maintaining it in two places means we can create accidental differences between the two.

The code from CUB should be directly (albeit through nvrtc) reused in c.parallel.