Fixed multiple bugs in hashmap implementation.

1. Fixed infinite loop issue that occurred when map was full of
   tombstones;
2. Fixed bug related to tombstone count decrement when adding a new
   key on a deleted slot;
3. Added proper map resize when keys are added or removed;
4. Fixed inconsistent error messages;
5. Added proper NULL check on map_remove method.

docs/map.md

@@ -54,6 +54,7 @@ defined as follows:
 typedef enum {
     MAP_OK = 0x0,
     MAP_ERR_ALLOCATE,
+    MAP_ERR_OVERFLOW,
     MAP_ERR_INVALID,
     MAP_ERR_NOT_FOUND
 } map_status_t;