Sqrt(X) and C-Sqrt(X) gates

[Cryoris]

Summary

Add the Sqrt(X) (as SX) and C-Sqrt(X) (as CSX) gates and closes #4637.

Details and comments

Uses the decompositions

sx q0 {rz(-pi/2) q0; h q0; rz(-pi/2) q0;}   # contains global phase
csx q0, q1 {h q1; cu1(pi/2) q0, q1; h q1;}

[ajavadia]

In the equivalence library there is an entry for u3 in terms of rx(pi/2) (in u3_qasm_def). Can you create another entry for this but this time with sx?

[Cryoris]

Should we add a SXdgGate for this? Probably yes since all of the basic single qubit gates have inverses.

[ajavadia]

Yes we should. sx is rx(pi/2) and sxdg is rx(-pi/2).
(This is where v and vdg start to look better :) )