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)
• [x] 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
• [x] 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 https://github.com/AccelerateHS/accelerate-llvm/pull/75