Printing a string wrote each character with a separate call, one
fwrite() per character, or one jv_string_append_buf() for tojson and
tostring, which made output of string-heavy data slow.
Scan ahead for the next byte that needs escaping and write the run of
bytes before it with a single call, without changing the output.
Printing strings is now significantly faster, most of all for long
strings with few characters to escape, and the same applies to tojson
and tostring. With --ascii-output, text that is almost all non-ASCII is
about the same speed, since nearly every byte is escaped anyway.
Michiel van Slobbe