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/stacks.c

@@ -459,7 +459,7 @@ SCM_DEFINE (scm_stack_length, "stack-length", 1, 0, 0,
 void
 scm_init_stacks ()
 {
-  scm_sys_stacks = scm_make_fluid ();
+  scm_sys_stacks = scm_make_thread_local_fluid (SCM_BOOL_F);
   scm_c_define ("%stacks", scm_sys_stacks);
   
   scm_stack_type = scm_make_vtable (scm_from_locale_string (SCM_STACK_LAYOUT),