Avoid stacks in dynamically-bound values

* libguile/dynstack.h:
* libguile/dynstack.c (scm_dynstack_find_old_fluid_value): New
  function.
* libguile/fluids.c (saved_dynamic_state_ref): New helper.
  (scm_fluid_ref): Fix docstring.
  (scm_fluid_ref_star): New function allowing access to previous values
  for a fluid.
  (scm_dynamic_state_ref): New internal function.
* libguile/fluids.h: Add scm_fluid_ref_star and scm_dynamic_state_ref.
* libguile/stacks.c (scm_stack_id): Adapt to %stacks not being a chain.
* libguile/throw.c (catch, throw_without_pre_unwind): Adapt to
  %exception-handlers not being a chain.
* module/ice-9/boot-9.scm (catch, dispatch-exception): Instead of having
  %exception-handlers be a chain, use fluid-ref* to access the chain
  that is in place at the time the exception is thrown.  Prevents
  unintended undelimited capture of the current exception handler stack
  by a delimited "catch".
  (%start-stack): Similarly, don't be a chain.
* module/system/repl/debug.scm (frame->stack-vector):
* module/system/repl/error-handling.scm (call-with-error-handling):
* module/ice-9/save-stack.scm (save-stack): Adapt to %stacks not being a
  chain.
* test-suite/tests/exceptions.test ("delimited exception handlers"): Add
  tests.
* doc/ref/api-control.texi (Fluids and Dynamic States): Add docs.

libguile/dynstack.c

@@ -504,6 +504,55 @@ scm_dynstack_find_prompt (scm_t_dynstack *dynstack, SCM key,
   return NULL;
 }
 
+SCM
+scm_dynstack_find_old_fluid_value (scm_t_dynstack *dynstack, SCM fluid,
+                                   size_t depth, SCM dflt)
+{
+  scm_t_bits *walk;
+
+  for (walk = SCM_DYNSTACK_PREV (dynstack->top); walk;
+       walk = SCM_DYNSTACK_PREV (walk))
+    {
+      scm_t_bits tag = SCM_DYNSTACK_TAG (walk);
+
+      switch (SCM_DYNSTACK_TAG_TYPE (tag))
+        {
+        case SCM_DYNSTACK_TYPE_WITH_FLUID:
+          {
+            if (scm_is_eq (WITH_FLUID_FLUID (walk), fluid))
+              {
+                if (depth == 0)
+                  return SCM_VARIABLE_REF (WITH_FLUID_VALUE_BOX (walk));
+                else
+                  depth--;
+              }
+            break;
+          }
+        case SCM_DYNSTACK_TYPE_DYNAMIC_STATE:
+          {
+            SCM state, val;
+
+            /* The previous dynamic state may or may not have
+               established a binding for this fluid.  */
+            state = scm_variable_ref (DYNAMIC_STATE_STATE_BOX (walk));
+            val = scm_dynamic_state_ref (state, fluid, SCM_UNDEFINED);
+            if (!SCM_UNBNDP (val))
+              {
+                if (depth == 0)
+                  return val;
+                else
+                  depth--;
+              }
+            break;
+          }
+        default:
+          break;
+        }
+    }
+
+  return dflt;
+}
+
 void
 scm_dynstack_wind_prompt (scm_t_dynstack *dynstack, scm_t_bits *item,
                           scm_t_ptrdiff reloc, scm_i_jmp_buf *registers)