ToC method
Converts value to its C representation into a tracked slot, or returns
an existing slot if value is null.
The slot key defaults to 'default'. Equivalent to calling
value.toC(temp, key) directly.
Implementation
Pointer<T> ToC([X? value, String? key]) {
key = _slotKey(key);
return value?.toC(_temp, key) ?? At(key);
}