🎨 Return registers from `allocQubitRegister()` methods

[denialhaag]

Currently, QC and QCO's allocQubitRegister() methods allocate a register, load all qubits, and then return a SmallVector of the qubits.

Instead, the methods should only allocate the register and then return a structure that can be indexed to load the qubits. Another method that loads all qubits might also make sense.

For more details, see #1542 (comment), #1580 (comment), and #1580 (comment).

[denialhaag]

After some discussion in #1627, we have decided against lazy loading. #1627 will close this issue.