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:

• [x] Double-free
• [x] Double-free, delayed
• [x] Double-free, interleaved
• [x] Invalid free
• [x] 1-byte overflow
• [x] 1-byte underflow
• [x] 1Mb overflow
• [x] 1Mb underflow
• [x] Read-after-free
• [x] Write-after-free
• [x] Read of a zero-sized allocation
• [x] Write of a zero-sized allocation
• [x] Unaligned free
• [x] Uninitialized free
• [x] Sized delete in C++
• [x] Guarded memcpy/memset

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

[jvoisin]

See https://github.com/daanx/mimalloc-bench/tree/master/bench/security