PERL_DEBUG_READONLY_OPS with the new allocator

PERL_DEBUG_READONLY_OPS with the new allocator

I want to eliminate the old slab allocator (PL_OP_SLAB_ALLOC),
but this useful debugging tool needs to be rewritten for the new
one first.

This is slightly better than under PL_OP_SLAB_ALLOC, in that CVs cre-
ated after the main CV starts running will get read-only ops, too.  It
is when a CV finishes compiling and relinquishes ownership of the slab
that the slab is made read-only, because at that point it should not
be used again for allocation.

BEGIN blocks are exempt, as they are processed before the Slab_to_ro
call in newATTRSUB.  The Slab_to_ro call must come at the very end,
after LEAVE_SCOPE, because otherwise the ops freed via the stack (the
SAVEFREEOP calls near the top of newATTRSUB) will make the slab writa-
ble again.  At that point, the BEGIN block has already been run and
its slab freed.  Maybe slabs belonging to BEGIN blocks can be made
read-only later.

Under PERL_DEBUG_READONLY_OPS, op slabs have two extra fields to
record the size and readonliness of each slab.  (Only the first slab
in a CV’s slab chain uses the readonly flag, since it is conceptually
simpler to treat them all as one unit.)  Without recording this infor-
mation manually, things become unbearably slow, the tests taking hours
and hours instead of minutes.