Fix and speed up en/decoding of UTF-8 strings

Fix and speed up en/decoding of UTF-8 strings

* src/coding.c (get_char_bytes, encode_string_utf_8)
(decode_string_utf_8): Fix commentary.
(encode_string_utf_8): Return the original ASCII string only
if NOCOPY is non-zero.
(decode_string_utf_8): Accept 2 additional arguments STR and
STR_LEN, which allow to pass the input text as a C string.
(make_string_from_utf8): Delegate the job to decode_string_utf_8.
* src/coding.h: Update the prototype of decode_string_utf_8.
* src/json.c (json_encode): Call encode_string_utf_8.