QMaskFusion

[WrathfulSpatula]

This adds a new gate fusion layer, which will typically sit as a layer just above QPager or QEngine. The inspiration was this paper, "Faster Schrödinger-style simulation of quantum circuits":

https://arxiv.org/abs/2008.00216

To paraphrase, one of the optimizations emphasized in that paper is making sure to pad gates with as few identity operations as possible, to avoid redundant traversals of the state vector. To this end, we add XMask(), YMask(), and ZMask(), which can efficiently apply multiple Pauli gates at once on different qubit indices, with a single state vector traversal. To take full advantage of this optimization, particularly in the default "optimal" layer stack, we insert the QMaskFusion layer at the appropriate level of the stack, to coalesce and fuse Pauli operators.

This does not cover multi-qubit-gate-fusion, yet, except for parallel Pauli operations. Another fusion layer, potentially closer in design to the original QFusion, might sit above QUnit, ultimately.

[WrathfulSpatula]

I'm trying benchmarks quickly, before merging, but I'm probably not going to be too unpleasantly surprised.

[WrathfulSpatula]

This is still unstable, in the benchmark suite.

[WrathfulSpatula]

The remaining bug was in QHybrid. We failed to match GPU/CPU modes, in QHybrid::ShuffleBuffers(), if the modes of the two engines to shuffle started mismatched.