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 '@'

Add execution time to tests

Make printf optional based on sprintf

Tests are executed based on available features

Remove strlen in array

Simplify. Should not use strlen() as possible. It needs complexity.