set_string_native

[Leopold Toetsch]

This vtable currently copies the string. I've tried now:

  PMC_str_val(SELF) = value;

instead (perlstring.pmc:193). This breaks exactly one test which 
explicitely tests this behavior nothing else.

And there is of course the assign opcode, that does the same. But we 
could have:

  set P0, "xx"       # ref
  assign P0, "xx"    # copy
This would need another vtable though,

Comments welcome,
leo