QPY dump and load in Rust

[gadial]

Summary

This PR adds implementations of the qpy module dump and load functions with Rust, making it possible to have a full QPY flow directly in Rust without Python.

Details and comments

This PR adds a top level QPYFile struct to the rust QPY framework, handling the general file metadata and the the circuits. A new file, interface.rs, handles the actual dump and load functions. We have python wrappers that are called from python's interface.py for supported versions.

Since we still don't have a pure rust QuantumCircuit, the rust functions dump_qpy and load_qpy still have Python remnants in their I/O. In particular, load_qpy returns python circuits (as Py<PyAny>) and dump_qpy accepts the QuantumCircuitData struct which is used as a transition layer from python's QuantumCircuit.

The circuit offset table, present in QPY17, is actually not required at all since all the size data for a given circuit is included in its serialization. Moreover, computing the circuit offset table is not trivial in the binrw framework. So to avoid complicating our data structures with a field that will become obsolete in QPY18, I added another struct, QPY17File. When converting QPYFile to QPY17File the circuits are serialized, the offset table is correctly calculated, and then QPY17File can also be written using the standard binrw macros.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core
• @mtreinish

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core
• @mtreinish