Use cheaper string constructors

Use cheaper string constructors

* src/coding.c (make_string_from_utf8):
* src/fileio.c (file_name_directory):
* src/lread.c (Fintern):
* src/gnutls.c (Fgnutls_format_certificate):
Use `make_multibyte_string` and `make_unibyte_string` instead of
more expensive `make_specified_string` and `make_string_from_bytes`.

(cherry picked from commit 7e358aa706f9dbfce24304e8a05904538615537f)