Move StandardGate definition into its own module

This is a staging commit, splitting up the giant monolithic
`qiskit_circuit::operations` to put `StandardGate` in its own module.
The code is unchanged, other than imports.

This is preparatory for moving more code out of `quantum_info` that
connects `StandardGate` to base `quantum-info` objects.

Author: jakelishman

crates/circuit/src/lib.rs

@@ -38,6 +38,7 @@ pub mod parameter;
 pub mod parameter_table;
 pub mod register_data;
 pub mod slice;
+pub mod standard_gate;
 pub mod util;
 pub mod var_stretch_container;
 mod variable_mapper;