All SV memory management code now inside the PMCs

All the SV memory management code is now contained within the PMCs.
The perl core code now has no knowledge of how different SV types get and
free memory, or how their internal structure is laid out.

This means that it's possible to change the internal structure of the different
PMC types on a PMC by PMC basis. However, there's not yet much benefit in doing
this, as all types larger than SvNULL need 3 or more integers of storage, which
is more than fits in an plain PMC.

To get further on restructuring PMC storage, either reference counting needs to
be abolished in favour of stack walking, GC, and the parrot high priority DOD,
or the SvFLAGS PMC implementation needs to be modified so that the simpler SV
types (IV, NV and RV) only store their flags in the PMC's private flag space,
and fake up full Perl 5 flags on demand.

I'm not sure which is actually best to do first. The roadmap has moving
string and number conversion code ahead of either of these, so I'll tackle
that first.

Nicholas Clark