Qrack v8 API

[WrathfulSpatula]

Qrack v8 makes two major changes to the API:

• Already merged in v7, qubit index parameters are checked for validity against simulators' allocated qubit bounds, to prevent OpenCL segmentation fault errors from explicitly invalid arguments.
• The content of this PR is to replace the public API parameter pattern "pointer, pointerLength" with std::vector.

After 5 years of holding out for the more C-like version of the interface, there's very little to be said in favor of "pointer, pointerLength" over std::vector, which effectively enforces bounds safety. To use a contiguous subset of a std::vector from user code space, one can still use the appropriate iterator-based std::vector constructor. This probably requires the system to create a copy of the subset, but at most this is likely fewer than 50 elements copied, typically more like 1 or 2 elements. The benefits of bounds-enforcement have been assessed to be far more important than the additional execution time overhead, if that overhead can even be detected.

I had loosely planned for v8 to shape up by end of calendar year, but I suspect that not much damage will be done by merging sooner, particular since the v7 commit history is obviously still available. The XACC plugin update is also ready-to-go, right here, with plans to open a PR for it once Qrack v8 is merged in the Qrack repository:

https://github.com/WrathfulSpatula/xacc/tree/qrack_8_api

If you need support in converting your user code to API v8, please don't hesitate to reach out with a GitHub issue on this repository!

[WrathfulSpatula]

This also closes #977.

[WrathfulSpatula]

Documentation updates will follow by the end of this weekend.