Add support for a two-Qubit swap gate

[brunorijsman]

Add support for a two-Qubit swap gate (not to be confused with an entanglement swap)

The Qubit class defined in netqasm.sdk.qubit currently supports the CNOT and CZ two-qubit gates.

The SWAP two-qubit gate is not natively supported by NetQASM.

As a temporary work-around, we can construct a SWAP gate out of three CNOT gates as described here: https://algassert.com/post/1717

[aromanro]

'mov' is actually a swap, from looking over the code.