Add a parameter to Perl_get_arena() to pass in the SV type, and record

Add a parameter to Perl_get_arena() to pass in the SV type, and record
this in the arena description. Change all sizes to unsigned values.
Make Perl_sv_free_arenas() loop downwards to free memory, simplifying
the logic. Remove my erroneous comment added in change 29881.

p4raw-id: //depot/perl@29882