Fix a recent change in coding.c

Fix a recent change in coding.c

This partially reverts the changes in "extern function cleanup".
* src/coding.c (encode_string_utf_8, decode_string_utf_8): Now
extern again.  They should NOT be static, as they are intended
to be used by the likes of json.c, where we need
highly-optimized code for processing UTF-8 strings.  E.g.,
decode_string_utf_8 beats make_string_from_utf8 by a factor of
2 to 5 in a large number of scenarios.