Implement a few "pair map" functions, mostly used for hash tables.

In order to efficiently check if nodes are contained within a
given set, without knowing exactly how many nodes there are and
allocating a ton of memory, it's probably best to use a hash table
(sparse sets would be more efficient but would require to allocate
all of their space at once). It's also pointless to reimplement most
of the hash table functionality from libds just to get it to work
with pairs (pattern id + node id), so pairmap.h provides a few
defaults that can be plugged into the default ht_s, as well as a
few macros to facilitate some hash table operations.

src/pairmap.c

@@ -0,0 +1,29 @@
+#include "pairmap.h"
+#include <stdlib.h>
+
+unsigned long pairmap_hash_func(void* key){
+  pairmap_key* new_key = key;
+  unsigned int hash = 0;
+  hash |= ((new_key->x & 0xffff) << 8) + (new_key->y & 0xffff);
+  return hash;
+}
+int pairmap_cmp_func(void* a, void* b){
+  pairmap_key* new_key_a = a;
+  pairmap_key* new_key_b = b;
+  return (new_key_a->x == new_key_b->x) && (new_key_a->y == new_key_b->y);
+}
+void* pairmap_copy_func(void* key){
+  pairmap_key* copy_from = key;
+  pairmap_key* new_key = malloc(sizeof(*new_key));
+  if(new_key){
+    new_key->x = copy_from->x;
+    new_key->y = copy_from->y;
+  }
+  return new_key;
+}
+void pairmap_init_ht(ht* ht){
+  ht_init(ht);
+  ht->hash_func = pairmap_hash_func;
+  ht->cmp_func = pairmap_cmp_func;
+  ht->copy_func = pairmap_copy_func;
+}