Use a correct and transparent definition of "code unit" in C code

[markshannon]

We should define _Py_CODEUNIT properly without the need for type punning.

Currently _Py_CODEUNIT is define as typedef 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:

typedef union {
    int16_t align;
    struct {
         uint8_t opcode;
         uint8_t oparg;
    };
} _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

[iritkatriel]

Looks good, would it be a drop-in replacement?

[markshannon]

Depends on how you define "drop-in"