(concat): Move the test for all nil in `append'

(concat): Move the test for all nil in `append'
back to the right place.
Always compute result_len_byte as for a multibyte string.  Then,
if the result will be single-byte, don't use that computation.
When converting single-byte to multibyte, don't use copy_text.
Do use copy_text between single-byte string and multibyte string.

(Fstring_make_multibyte, Fstring_make_unibyte): New functions.
(syms_of_fns): defsubr them.

(string_char_to_byte): Count from the beginning or the end,
whichever is closer.  Keep a cache of recent values.
(string_byte_to_char): Likewise.

(Fstring_lessp): Compare the strings char by char.