S_sv_pos_u2b_cached now updates the UTF-8 length cache if at the end of string.

S_sv_pos_u2b_cached now updates the UTF-8 length cache if at the end of string.

Pass in a boolean to S_sv_pos_u2b_forwards, which sets it to true if it
discovers that the UTF-8 offset is at (or after) the end of the string.
This can only happen if we don't already know the SV's length (in Unicode
characters), because if we know it, we always call S_sv_pos_u2b_midway().