Introducing the mrb_protect_raw() API function

The purpose is two-fold:

 1. to be able to specify a pointer directly when user data is used

    When using `mrb_protect()`, it is necessary to allocate objects by `mrb_obj_cptr()` function when using user data.

    Adding `mrb_protect_raw()` will make it simpler to reimplement `mrbgems/mruby-error`.

 2. to correctly unwind callinfo when an exception is raised from a C function defined as a method (the main topic)

    If a method call is made directly under `mrb_protect()` and a C function is called, control is returned from `mrb_protect()` if an exception occurs there.
    In this case, callinfo is not restored, so it is out of sync.
    Moreover, returning to mruby VM (`mrb_vm_exec()` function) in this state will indicate `ci->pc` of C function which is equal to `NULL`, and subsequent `JUMP` will cause `SIGSEGV`.

    Following is an example that actually causes `SIGSEGV`:

    - `crash.c`

      ```c
      #include <mruby.h>
      #include <mruby/compile.h>
      #include <mruby/error.h>

      static mrb_value
      level1_body(mrb_state *mrb, mrb_value self)
      {
        return mrb_funcall(mrb, self, "level2", 0);
      }

      static mrb_value
      level1(mrb_state *mrb, mrb_value self)
      {
        return mrb_protect(mrb, level1_body, self, NULL);
      }

      static mrb_value
      level2(mrb_state *mrb, mrb_value self)
      {
        mrb_raise(mrb, E_RUNTIME_ERROR, "error!");
        return mrb_nil_value();
      }

      int
      main(int argc, char *argv[])
      {
        mrb_state *mrb = mrb_open();
        mrb_define_method(mrb, mrb->object_class, "level1", level1, MRB_ARGS_NONE());
        mrb_define_method(mrb, mrb->object_class, "level2", level2, MRB_ARGS_NONE());
        mrb_p(mrb, mrb_load_string(mrb, "p level1"));
        mrb_close(mrb);
        return 0;
      }
      ```

    - compile & run

      ```console
      % `bin/mruby-config --cc --cflags --ldflags` crash.c `bin/mruby-config --libs`

      % ./a.out
      zsh: segmentation fault (core dumped)  ./a.out
      ```

    After applying this patch, it will print exception object and exit normally.

The `mrb_protect()`, `mrb_ensure()` and `mrb_rescue_exceptions()` in `mrbgems/mruby-error` have been rewritten using `mrb_protect_raw()`.

src/vm.c

@@ -306,6 +306,49 @@ cipop(mrb_state *mrb)
   return c->ci;
 }
 
+MRB_API mrb_value
+mrb_protect_raw(mrb_state *mrb, mrb_protect_raw_func *body, void *userdata, mrb_bool *error)
+{
+  struct mrb_jmpbuf *prev_jmp = mrb->jmp;
+  struct mrb_jmpbuf c_jmp;
+  mrb_value result = mrb_nil_value();
+  int ai = mrb_gc_arena_save(mrb);
+  const struct mrb_context *c = mrb->c;
+  int ci_index = c->ci - c->cibase;
+
+  if (error) { *error = FALSE; }
+
+  MRB_TRY(&c_jmp) {
+    mrb->jmp = &c_jmp;
+    result = body(mrb, userdata);
+    mrb->jmp = prev_jmp;
+  }
+  MRB_CATCH(&c_jmp) {
+    mrb->jmp = prev_jmp;
+    result = mrb_obj_value(mrb->exc);
+    mrb->exc = NULL;
+    if (error) { *error = TRUE; }
+    if (mrb->c == c) {
+      while (c->ci - c->cibase > ci_index) {
+        cipop(mrb);
+      }
+    }
+    else {
+      // It was probably switched by mrb_fiber_resume().
+      // Simply destroy all successive CI_ACC_DIRECTs once the fiber has been switched.
+      c = mrb->c;
+      while (c->ci > c->cibase && c->ci->acc == CI_ACC_DIRECT) {
+        cipop(mrb);
+      }
+    }
+  }
+  MRB_END_EXC(&c_jmp);
+
+  mrb_gc_arena_restore(mrb, ai);
+  mrb_gc_protect(mrb, result);
+  return result;
+}
+
 void mrb_exc_set(mrb_state *mrb, mrb_value exc);
 static mrb_value mrb_run(mrb_state *mrb, const struct RProc* proc, mrb_value self);