Find the byte offset of the previous code point in string, before
C]*pos] does not have to be at the beginning of a code point.
Returns true on success, then value pointed to by C]pos] will be
updated to the found offset.
Otherwise returns false if C]*pos] was already at the end of the
string, then C]*pos] is unmodified.
This function just looks for an appropriate byte; it doesn[aq]t check
if found offset is the beginning of a valid code point.
If you are working with possibly invalid UTF-8 strings then it could
skip over some invalid bytes.