Extend ZMask() to PhaseRootNMask()

[WrathfulSpatula]

There's an obvious extension of ZMask() to "masked" S, T, and general PhaseRootN gates.ZMask() calculates parity of each masked permutation, and applies a -1 multiplicative factor if the parity is odd, achieving a batched set of masked Pauli Z gates. Similarly, for a general batched PhaseRootN gate, the effective extension of this parity calculation is to a ring with 2^(n+1) phase poles for the n-th root of -1, for a phase factor. (I can explain this in simpler terms, if a volunteer developer wants clarification.)

QEngine types could therefore be generally extended to arbitrary PhaseRootN gate fusion and batching, (along with Pauli X, which it already has). I don't think I'm going to personally take up this implementation immediately, but I might look at it by the weekend.

[WrathfulSpatula]

See the PR #874, for background.

[Vikrant-Khedkar]

Hey @WrathfulSpatula is this still open

[WrathfulSpatula]

@Vikrant-Khedkar It is! Another participant seemed interested, but they haven't followed up. Regardless, the bounty is up-for-grabs!

[jpacold]

Commenting to link #1009 with this.

[WrathfulSpatula]

Thank you, @jpacold!