test_m17n.rb: split test_scrub

	* test/ruby/test_m17n.rb (TestM17N#test_scrub): split into some
  tests.
	* include/ruby/ruby.h: add raw FL macros, which assume always the
	  argument object is not a special constant.

	* internal.h (STR_EMBED_P, STR_SHARED_P): valid only for T_STRING.

	* string.c: deal with taint flags directly across String instances.

	* transcode.c (rb_econv_substr_append, econv_primitive_convert):
	  the result should be infected by the original string.

	* string.c (rb_str_scrub): the result should be infected by the
	  original string.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@54260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e