Add isc_hashmap API that implements Robin Hood hashing

Add isc_hashmap API that implements Robin Hood hashing

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().