string.c: rb_str_new_static

* string.c (rb_str_new_static): create string object with static
  buffer.  incorporated from mruby.

* string.c (rb_{usascii,utf8,enc}_str_new_static): ditto with
  encodings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47639 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
intern.h: rb_str_new_literal

* include/ruby/intern.h (rb_str_new_literal): make ruby string
  from literal C-string.  incorporated from mruby.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47640 b2dd03c8-39d4-4d8f-98ff-823fe69b080e