Closes#1234
multiqueue:
- Implement multiqueue_size()
- Rename MultiQueueItem.parent to MultiQueueItem.parent_item, to avoid confusion
with MultiQueue.parent.
Changes:
1. Linter finds no errors now.
2. Most of macros changed to `static inline … FUNC_ATTR_ALWAYS_INLINE` functions
(that was the purpose: they are easier to debug).
3. Queue is now not a pair of void* pointers, but a struct with two QUEUE
pointers, next and prev. This should not affect anything, except that _QUEUE
private macros can really be avoided without reducing readability and they do
not need any casts.
