Add thread local fluids

* libguile/fluids.h (struct scm_dynamic_state): Add thread_local_values
  table.  Thread locals are flushed to a separate thread-local table.
  The references are strong references since the table never escapes the
  thread.
  (scm_make_thread_local_fluid, scm_fluid_thread_local_p): New
  functions.
* libguile/fluids.c (FLUID_F_THREAD_LOCAL):
  (SCM_I_FLUID_THREAD_LOCAL_P): New macros.
  (restore_dynamic_state): Add comment about precondition.
  (save_dynamic_state): Flush thread locals.
  (scm_i_fluid_print): Print thread locals nicely.
  (new_fluid): Add flags arg.
  (scm_make_fluid, scm_make_fluid_with_default, scm_make_unbound_fluid):
  Adapt.
  (scm_make_thread_local_fluid, scm_fluid_thread_local_p): New
  functions.
  (fluid_set_x): Special flushing logic for thread-locals.
  (fluid_ref): Special cache miss logic for thread locals.
* libguile/stacks.c (scm_init_stacks):
* libguile/throw.c (scm_init_throw): %stacks and %exception-handler are
  thread-locals.
* libguile/threads.c (guilify_self_2): Init thread locals table.
* test-suite/tests/fluids.test ("dynamic states"): Add test.
* doc/ref/api-control.texi (Fluids and Dynamic States): Add link to
  Thread-Local Variables.
* doc/ref/api-scheduling.texi (Thread Local Variables): Update with real
  thread-locals.
* NEWS: Update.

libguile/threads.c

@@ -458,6 +458,7 @@ guilify_self_2 (SCM dynamic_state)
   }
 
   t->dynamic_state = scm_gc_typed_calloc (scm_t_dynamic_state);
+  t->dynamic_state->thread_local_values = scm_c_make_hash_table (0);
   scm_set_current_dynamic_state (dynamic_state);
 
   t->dynstack.base = scm_gc_malloc (16 * sizeof (scm_t_bits), "dynstack");