Support for OpenQASM 3.0 classical bits and registers

[burgholzer]

The new OpenQASM standard introduces many additional datatypes besides quantum and classical registers. This allows to describe classical components of quantum algorithms in a very intuitive way. As a first step, support for all kinds of new types should be added to the QFR library and, correspondingly, to the QuantumComputation class. For details on all the new typed provided by the standard, see https://qiskit.github.io/openqasm/language/classical.html.

Classical bits and registers could previously just be used as the target of a measurement and (in a very limited fashion) as the classical control in an if statement. The new OpenQASM 3.0 standard enriches these capabilities by allowing to:

• assign bitstrings to classical registers, e.g., bit[8] a = "10001111";
• execute bitwise operations such as and (&), or |, xor ^ and the corresponding assignment operators &=, |=, ^=
• left and right shift << and >>, as well as <<= and >>= by an unsigned integer
• not ~
• circular shifts rotl and rotr

First of all, the QuantumComputation class needs to be capable of allocating and maintaining classical registers.
Most certainly, the manipulation of these registers will require the introduction of a new kind of operation, e.g., ClassicalOperation.
Such an operation will not be unitary. As a consequence, the concept of the NonUnitaryOperation as it is now may need to be revisited/refactored.

Depends on #28 for the availability of the bit keyword in the parser.