Simplify backtrace mechanism; fix #3633 #3634 #3644

Instead of preserving a backtrace in `mrb_state`, `mrb_exc_set`
keeps packed backtrace in an exception object. `#backtrace` unpacks
it to an array of strings.

Author: matz

include/mruby.h

@@ -152,12 +152,6 @@ struct mrb_context {
 
 struct mrb_jmpbuf;
 
-typedef struct {
-  const char *filename;
-  int lineno;
-  mrb_sym method_id;
-} mrb_backtrace_entry;
-
 typedef void (*mrb_atexit_func)(struct mrb_state*);
 
 #define MRB_STATE_NO_REGEXP 1
@@ -172,15 +166,9 @@ typedef struct mrb_state {
 
   struct mrb_context *c;
   struct mrb_context *root_c;
+  struct iv_tbl *globals;                 /* global variable table */
 
   struct RObject *exc;                    /* exception */
-  struct {
-    struct RObject *exc;
-    int n;
-    int n_allocated;
-    mrb_backtrace_entry *entries;
-  } backtrace;
-  struct iv_tbl *globals;                 /* global variable table */
 
   struct RObject *top_self;
   struct RClass *object_class;            /* Object class */

include/mruby/error.h

@@ -25,7 +25,6 @@ MRB_API void mrb_sys_fail(mrb_state *mrb, const char *mesg);
 MRB_API mrb_value mrb_exc_new_str(mrb_state *mrb, struct RClass* c, mrb_value str);
 #define mrb_exc_new_str_lit(mrb, c, lit) mrb_exc_new_str(mrb, c, mrb_str_new_lit(mrb, lit))
 MRB_API mrb_value mrb_make_exception(mrb_state *mrb, int argc, const mrb_value *argv);
-MRB_API mrb_value mrb_exc_backtrace(mrb_state *mrb, mrb_value exc);
 MRB_API mrb_value mrb_get_backtrace(mrb_state *mrb);
 MRB_API mrb_noreturn void mrb_no_method_error(mrb_state *mrb, mrb_sym id, mrb_value args, const char *fmt, ...);