fix: cast node.size to IntegerLiteral for qubit register size

[rmshaffer]

Issue #, if available:
#240

Description of changes:
Starting with the changes from #258 and performing the additional work required to merge to main.

The biggest outstanding items are:

• Refactor the new logic in functions.py into a separate helper function, for readability. See this comment thread.
• Expand unit tests to ensure correctness of implementation. See this comment thread.
• Discuss among maintainers to ensure that the new behavior is actually in line with how we want to support OpenQASM in the default simulator (since the OpenQASM spec isn't entirely clear on all of this).

Testing done:
tox

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[EuGig]

@rmshaffer, @DanBlackwell
Maybe we should rethink the following case test:

bit[3] b;
qubit["10"] r1;
h r1["01" << "01"];

In this regard the spec explicitly states that shifting, either right or left, is supported by an unsigned integer (https://openqasm.com/language/classical.html#classical-bits-and-registers).
Shifting by a bitstring causes a strange behaviour in case its length does not match the length of the specified qbuit array:

bit[3] b;
qubit[3] r1;
h r1["01" << "01"];
IndexError: boolean index did not match indexed array along dimension 0; dimension is 3 but corresponding boolean dimension is 2

Since we are enabling registers sizes to be specified also as bitstring, maybe we should raise an error if the LHS is also a bitstring.
This influences the refactoring since, it pertains code regarding shifting operations.

[DanBlackwell]

Maybe slightly related, but I found the following here the other day:

I'm not sure whether this is implying that it is valid to cast an array of bits into int and vice versa; but in that case I think that when applying any ops (+ - << >> | & ^ etc) to an 'array of bits' (which a 'bit string literal' is afaik) the best approach is to cast to 'int' rather than reimplement them (and cast back if required). I'm not sure how integer over- and underflows are meant to be handled though.