Vector indexing and insertion operations

[HugoPeters1024]

Description
Allow indexing into the existing Vec primitive to enable branchless, constant time, memory reads with runtime offsets. Especially useful for having a local chunk of memory during an Exp computation that doesn't have to live in global memory.

Motivation and context
This functionality will bring Accelerate closer to supporting BVH traversal for realtime (ray/path)tracing with complex scenes.

How has this been tested?
It has not, this PR is in development and should not be merged right now :}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask. We're here to help!

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• Merge in lock step with Implement the vector indexing operation accelerate-llvm#75

[tmcdonell]

Remove VecPac and VecUnpack? Those just get turned your new index and write instructions. We might as well also add VecShuffle, corresponding to the shufflevector instruction.

[HugoPeters1024]

One of the more complicated things about VecPack and Unpack is the need to derive the tuple type. I got rather stuck trying to make changes to that whole pipeline, could you assist me on this live sometime?

[tmcdonell]

Ah, good point. Okay let's just shelve that for now.

I've been thinking how this vector support can be restructured after you mentioned you couldn't have a Vec Node (or any non-primitive thing), which is sort of a weird limitation for our language...

[HugoPeters1024]

The two blocking constraints are Prim Node (which can be implemented I think). However EltR Node ~ Node is the culprit. Perhaps some unsafe coerceon from and to a flattened byte vector?