json_escape: add json_escape_unescape_len()

json_escape: add json_escape_unescape_len()

Lacking a variant of json_escape_unescape() that takes a sized span,
the common case of unescaping a JSON string that is embedded inside a
larger buffer is forced to make a copy of the escaped string so as to
terminate it with a NUL character. This extra copy can be avoided if
we can pass an explicit length to the unescaping function.

Also, a drive-by fix: shrink the returned unescaped string to fit its
contents, which can save the caller a subsequent call to strlen().