Re-implement the SvOOK() hack to store the offset as a BER encoded

Re-implement the SvOOK() hack to store the offset as a BER encoded
number in the part of the PVX that is being released. (It will always
fit, as chopping off 1 byte gives just enough space for recording a
delta of up to 127). This allows SvOOK() to co-exist with SvIOK_on(),
which means all the calls to SvOOK_off() [with the possibility of a
call to sv_backoff()] in SvIOK_on() can be removed. This ought to make
a lot of straight line code a little bit simpler.
OOK()d scalars can now be SVt_PV, as the IVX isn't needed.

p4raw-id: //depot/perl@32836