Remove implicit conversion using to_str method; fix #3854

We have added internal convenience method `__to_str` which
does string type check.

The issue #3854 was fixed but fundamental flaw of lack of stack
depth check along with fibers still remains. Use `MRB_GC_FIXED_ARENA`
for workaround.

include/mruby.h

@@ -854,10 +854,6 @@ typedef const char *mrb_args_format;
 /**
  * Retrieve arguments from mrb_state.
  *
- * When applicable, implicit conversions (such as `to_str`, `to_ary`, `to_hash`) are
- * applied to received arguments.
- * Used inside a function of mrb_func_t type.
- *
  * @param mrb The current MRuby state.
  * @param format [mrb_args_format] is a list of format specifiers
  * @param ... The passing variadic arguments must be a pointer of retrieving type.
@@ -1187,6 +1183,7 @@ MRB_API void mrb_gc_unregister(mrb_state *mrb, mrb_value obj);
 
 MRB_API mrb_value mrb_to_int(mrb_state *mrb, mrb_value val);
 #define mrb_int(mrb, val) mrb_fixnum(mrb_to_int(mrb, val))
+MRB_API mrb_value mrb_to_str(mrb_state *mrb, mrb_value val);
 MRB_API void mrb_check_type(mrb_state *mrb, mrb_value x, enum mrb_vtype t);
 
 typedef enum call_type {

include/mruby/string.h

@@ -311,8 +311,7 @@ MRB_API mrb_value mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb
  * @param [mrb_value] str Ruby string.
  * @return [mrb_value] A Ruby string.
  */
-MRB_API mrb_value mrb_string_type(mrb_state *mrb, mrb_value str);
-
+MRB_API mrb_value mrb_ensure_string_type(mrb_state *mrb, mrb_value str);
 MRB_API mrb_value mrb_check_string_type(mrb_state *mrb, mrb_value str);
 MRB_API mrb_value mrb_str_new_capa(mrb_state *mrb, size_t capa);
 MRB_API mrb_value mrb_str_buf_new(mrb_state *mrb, size_t capa);