string.c: rb_enc_str_new_cstr

* string.c (rb_enc_str_new_cstr): new function to create a string from
  the C-string pointer with the specified encoding.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42811 b2dd03c8-39d4-4d8f-98ff-823fe69b080e