Apply scalar context to state var init

6bd9b5cf6 did apply srefgen optimisations to \($a//=3), but not to
\(state $x = 3), despite what the commit message says, because the
‘once’ op does not go through S_new_logop with OP_ONCE as the type.
So this commit applies the scalar context afterwards, allowing
\(state $x = 3) to get a srefgen op, not a refgen.

(The OP_ONCE op is created via newCONDOP, which creates an
OP_COND_EXPR, which is then converted to an OP_ONCE.  I don’t under-
stand why it needs to be so complicated.  Maybe this could be
simplified.)
[perl #94484] Custom ops trip assertions in new*OP

Without allowing OP_CUSTOM in these functions, there is no way to cre-
ate a custom op while sticking to the API.
Change OP_SIBLING to OpSIBLING

to match the existing convention (OpREFCNT, OpSLAB).

Dave Mitchell asked me to wait until after his multideref work
was merged.

Unfortunately, there are now CPAN modules using OP_SIBLING.
Increase $IO::VERSION to 1.35
Increase $Devel::Peek::VERSION to 1.20