Fix str_format_len.

Add more kernel tests

Refactor hex_to_str().

Fix str_to_format.

Prevent memory leak when string literal is created.

Patch for inheritance of built-in class

Fix GC of struct

Modify Kernel#clone and Kernel#dup

Fix Struct#new and GC

Kernel#clone, Kernel#dup:
 - 'iv' should not be shared with the original object,
   but it should be copied.

Kernel#clone:
 - 'mt' of singleton_class should be copied.

Instances of Struct's subclass should have MRB_TT_STRUCT.

remove assertion for false assumption; mrb_str_new(0, len) where len > 0 can be used to allocate an unintialized string

Reduce mrb_str_new2

fixed attr_*: forgot '@'