Fix str_replace in string.c

Free iv of MRB_TT_DATA instance.

 - Increment refcnt of shared string.
 - Free buffer before replacing.
 - Clear MRB_STR_SHARED after decref is called.
 - Fix target of aux.capa.

Make strucdata optional if ENABLE_STRUCT is activated

Sprintf NotImplementedError

Supress "warning: array subscript has type ‘char’"

Use sizeof to get char array sizes.

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