Deprecate accidentally public internal helper methods on QuantumCircuit

[mtreinish]

What should we add?

The QuantumCircuit class has 5 methods which shouldn't really be exposed publically as they're only used as internal helpers as part of other functionality. However they have accidentally been documented as part of the public API in Qiskit and we need to support them for the remainder of 1.x. We should however mark these public entrypoints as deprecated ASAP and work towards making them internal only in 2.0. These methods are:

• .cast()
• .cbit_argument_conversion()
• .cls_instances()
• .cls_prefix()
• .qbit_argument_conversion()