Implements action of multi-controlled Pauli X gate.

Result is guaranteed to consist exclusively of 1-qubit, CNOT and CCNOT gates. If free_qubits has at least 1 element, result has lengts O(len(controls)).

controls - control qubits. targets - target qubits. free_qubits - qubits which are neither controlled nor target. Can be modified by algorithm, but will end up in their initial state.