We should define_Py_CODEUNIT properly without the need for type punning.

Currently_Py_CODEUNIT is define astypedef uint16_t _Py_CODEUNIT; but it really an 8 bit opcode followed a bit operand aligned to 16 bits. Which means we need to resort to type punning to access the operand and oparg individually.
E.g.https://github.com/python/cpython/blob/main/Include/cpython/code.h#L32

PEP 7 states that "Python 3.11 and newer versions use C11 without optional_features".
So let's use a union with anonymous struct to define it properly:

typedefunion {int16_talign;struct {uint8_topcode;uint8_toparg;    };}_Py_CODEUNIT;

@iritkatriel thoughts?

Linked PRs

• GH-100222: Redefine _Py_CODEUNIT as a union to clarify structure of code unit. #100223
• GH-100222: fix typo _py_set_opocde -> _py_set_opcode #100259