Generalizing GPU Indexing PR changes

[MrBurmark]

This is a capture of some of the discussion in our meeting on 1/10.
This refers to #1334.
Please add anything I forgot to mention.

• Break into separate PRs for forall and kernel changes
• Add separate policies for scan and sort because new forall policies contain thread mapping information that is not relevant to scan and sort. Though I suppose that it could be relevant if we ever implemented them ourselves.
• kernel policies can specify thread mapping without block mapping, but both should always be mapped.
  • use global with grid_size set to 1 to specify 1 block instead of just a thread policy.
• kernel launch policy can be inconsistent with use
  • require higher block size in the policies inside the launch.
• kernel policies can contradict each other leading to incorrect mapping behavior
  • This policy snippet would launch a kernel with block size 256 but the first for would map incorrectly because it expected a block size of 128, for< direct_thread_x<128>, lambda<0> >, for< direct_thread_x<256>, lambda<0> >
• optimize policies with grid_size or block_size set to 1, index is always 0

[rhornung67]

This covers everything I can think of. Thanks.

[rhornung67]

@MrBurmark can this be closed?