Refactor the JIT's type-lowering/specialization pass's data structures.

[markshannon]

Currently the symbol structure looks like this:

struct _Py_UopsSymbol {
    int flags;  // 0 bits: Top; 2 or more bits: Bottom
    PyTypeObject *typ;  // Borrowed reference
    PyObject *const_val;  // Owned reference (!)
    unsigned int type_version; // currently stores type version
};

which is both hard to use and inefficient. This awkwardness is effectively blocking 2 PRs: python/cpython#119478 and python/cpython#121002 as I am not willing to risk introducing bugs.

We should change the symbol to a tagged union:

union _Py_UopSymbol {
    uint8_t tag;
    _PyUopSym_Instance instance;
    _PyUopSym_Value value;
    _PyUopSym_Function func;
    _PyUopSym_Tuple tuple;
    ...
}

Each of the subtypes would start with uint8_t tag; and should be no larger than 16 bytes (on a 64bit machine).
E.g.

struct _PyUopSym_Instance {
    uint8_t tag;
    PyTypeObject *typ;  // Borrowed reference
};

All operations would take the form of a switch statement. Using an enum for the tag would allow the C compiler to generate warnings when a case was forgotten.
E.g.

bool
_Py_uop_sym_matches_type(_Py_UopsSymbol *sym, PyTypeObject *typ) 
{
    switch(sym->tag) {
        case INSTANCE_TAG:
             return sym->instance.typ == typ;
        case VALUE_TAG:
            return Py_TYPE(sym->value.value) == typ;
        ...

This is nothing novel, just good engineering practice.

[markshannon]

CPython issue: python/cpython#128939

[markshannon]

Done