remarks WRT clone

[Leopold Toetsch]

In former days and before YAPC::EU I changed the original clone vtable, 
which was IIRC:
  PMC* clone()   # return new clone of pmc
to the now existing form, which gets an uninitialized destination PMC. 
This change was at that time necessary because of reasons described in
F<docs/dev/infant.dev> keyword "Variant 2: Anchor early, anchor often".

This is solved, stackwalking during DOD works, so /me thinks, that we 
can use again the original signature of the clone vtable. This does also 
simplify switching clone to Parrot_clone (the real and final clone via 
freeze/thaw), which just happens to return a newly created PMC.

leo