Add robinhood hashing to structs.

This corrects changes in internal structure when values
were inserted in different orders (which was previously
incorrect.) Robinhood hashing should correct this by
making the internal structure of the hashtable invariant
of insertion order. This, in turn, allows naive and deterministic equality, comparison, and hashing of structs.

include/gst/gst.h

@@ -209,7 +209,8 @@ struct GstThread {
     enum {
         GST_THREAD_PENDING = 0,
         GST_THREAD_ALIVE,
-        GST_THREAD_DEAD
+        GST_THREAD_DEAD,
+        GST_THREAD_ERROR
     } status;
 };