GH-131498: Cases generator: Parse down to C statement level.

[markshannon]

This PR enhances the cases generator parser to parse instructions and labels down to C statement level.

This:

• Simplifies the rest of the code generator.
• Enables us to treat #if conditionals like normal if statements ensuring that the free-threading and default builds have the same properties and are properly understood by the code generator.

Because we are now parsing the source, rather than simply tokenizing it, the output no longer contains the comments present in the original.

• Issue: Top-of-stack caching in the interpreter #131498

[picnixz]

Maybe to reduce memory footprint, use slots=True

[markshannon]

I don't think it is worth it to save a few kbytes.