Hashtable: adjust shrink-on-remove factor

Due to the use of prime numbers Hashtable_remove used to never shrink
from some sizes. For example, a size 8191 hashtable would try to shrink
to 4095, which nextPrime would round back to 8191 instead of the
intended 4093. A factor of 3 is enough to allow every prime size used to
shrink to the previous one.

Hashtable.c

@@ -286,7 +286,7 @@ void* Hashtable_remove(Hashtable* this, ht_key_t key) {
 
    /* shrink on load-factor < 0.125 */
    if (8 * this->items < this->size)
-      Hashtable_setSize(this, this->size / 2);
+      Hashtable_setSize(this, this->size / 3); /* account for nextPrime rounding up */
 
    return res;
 }