Refactor PythonConstant types into a true union

[atifaziz]

This PR proposes to refactor PythonConstant types into a true union via a class hierarchy where PythonConstant becomes an abstract base class and the various types its nested subclasses. This has the following benefits:

• Each subclass has its own clear, simpler and overriding implementation of ToString instead of a switch-case.
• Each subclass holds its own strong-typed value, e.g. Value on PythonConstant.Integer is typed as long whereas Value on PythonConstant.String is typed as string.
• Each subclass only requires the storage space for its value instead of the sum of all possible values.
• Subclasses like Bool and None have statically initialised values, which avoids unnecessary allocations.
• PythonConstant.ConstantType is no longer needed.
• Subclasses enable simpler, stronger and more robust pattern-matching (see updates in ArgumentReflection.ArgumentSyntax).

[tonybaloney]

Thanks, this is a lot cleaner. I think we started with 2 constant types then it got out of hand quickly