Plan C for foiling the algorithmic complexity attack

Plan C for foiling the algorithmic complexity attack
(based on Chip's plan A (binary compatibility with 5.8.0 and 5.8.1),
 Chip's plan B (do something new inside the hv functions)
 and introspective sort)
Provides infrastructure for hashes to change their hash function
if necessary, and code in hsplit to detect pathalogical data and
instigate a random rehashing.
Needs refinement. Let's see how much smoke it creates.

p4raw-id: //depot/perl@21471