Add a security benchmark

[jvoisin]

It would be nice to have a security-related benchmark, not focused on memory consumption or speed, but on the ability of allocators to detect and neuter memory corruptions. For example, allocators should instantly abort on double-free.

List of tests to add, for large, small and medium allocations:

• Double-free
• Double-free, delayed
• Impossibly large malloc
• Invalid free
• Overflow
• Read-after-free
• Read of a zero-sized allocation
• Unaligned free
• Uninitialized free
• Write-after-free
• Write of a zero-sized allocation

We should take inspiration from hardened_malloc's testsuite, and shellphish's how2heap