Instead of marking the unused entities with UNUSED(x) macro in the
function body, use a `ISC_ATTR_UNUSED` attribute macro that expans to
C23 [[maybe_unused]] or __attribute__((__unused__)) as fallback.
In case, we are trying to hash the empty key into the hashmap, the key
is going to have zero length. This might happen in the unit test.
Allow this and add a unit test to ensure the empty zero-length key
doesn't hash to slot 0 as SipHash 2-4 (our hash function of choice) has
no problem with zero-length inputs.
Add new isc_hashmap API that differs from the current isc_ht API in
several aspects:
1. It implements Robin Hood Hashing which is open-addressing hash table
algorithm (e.g. no linked-lists)
2. No memory allocations - the array to store the nodes is made of
isc_hashmap_node_t structures instead of just pointers, so there's
only allocation on resize.
3. The key is not copied into the hashmap node and must be also stored
externally, either as part of the stored value or in any other
location that's valid as long the value is stored in the hashmap.
This makes the isc_hashmap_t a little less universal because of the key
storage requirements, but the inserts and deletes are faster because
they don't require memory allocation on isc_hashmap_add() and memory
deallocation on isc_hashmap_delete().
