Move to store thread join cond/lock/results directly

* libguile/threads.h: Add join data directly on the thread instead of
using a Scheme-side weak table.  It's less complicated and it will let
the weak table implementation use locks in Scheme; otherwise you would
have threads depending on weak tables and vice versa.
* libguile/threads.c (scm_trace_thread, guilify_self_1): Init and mark
the new members.
(thread_join_cond, thread_join_lock, thread_join_results)
(thread_init_joinable_x, thread_set_join_results_x): New accessors.
* module/ice-9/threads.scm (call-with-new-thread, join-thread): Use the
new accessors.

libguile/threads.h

@@ -123,6 +123,12 @@ struct scm_thread {
   SCM_STACKITEM *auxiliary_stack_base;
 #endif
 
+  /* For joinable threads, a cond to wait on joining, and a lock to
+     protect the results.  #f if not joinable.  */
+  SCM join_cond;
+  SCM join_lock;
+  SCM join_results;
+
   /* JIT state; NULL until this thread needs to JIT-compile something.  */
   struct scm_jit_state *jit_state;
 };