make PL_perlio an array of PerlIOl, not PerlIO *

make PL_perlio an array of PerlIOl, not PerlIO *

Layers in PerlIO are implemented as a linked list of PerlIOl structs;
eaxch one has a 'next' field pointing to the next layer. Now here's the
clever bit: When PerlIO* pointers are passed around to refer to a
particular handle, these are actually pointers to the 'next' field of the
*parent* layer (so to access the flags field say of a PerlIOl, you have to
double-defref it, e.g. (*f)->flags). The big advantage of this is that
it's easy for a layer to pop itself; when you call PerlIO_pop(f), f is a
pointer to the parent's 'next' field, so pop(f) can just do
*f = (*f)->next.

This means that there has to be a fake 'next' field above the topmost
layer. This is where PL_perlio comes in: it's a pointer to an arena of
arrays of pointers, each one capable of pointing to a PerlIOl structure.
When  a new handle is created, a spare arena slot is grabbed, and the
address of that slot is returned. This also allows for a handle with no
layers.

What this commit does is change PL_perlio from being an array of
PerlIO* into an array of PerlIOl structures - i.e. each element in the
array goes from being a single pointer, to having several fields. These
will be made used of in follow-up commits.