"copy ctor" benchmark doesn't scale linearly

[camel-cdr]

I was comparing my implementation against @ktprime's hash_map8, which uses almost the same copy implementation as my implementation (malloc + memcpy), but for some reason mine was >2 times slower.

hash_map8 allocated 32780176 bytes, while my implementation allocated 35651601 bytes, so there shouldn't be a 2x performance difference. (it's even worse when run on godbolt: https://godbolt.org/z/oqe4rY3qq)

After a bunch of testing and help from a friend, I figured out that the malloc mmap threshold (DEFAULT_MMAP_THRESHOLD_MAX) happens to be 33554432, which is exactly between the two sizes.

I don't think that this is a good idea to have this influence the benchmark that much, as the constants involved are arbitrary.

I'm not certain what the best way to fix this is, so I haven't written an PR yet.

We could bump the size of the elements to DEFAULT_MMAP_THRESHOLD_MAX, so every map is treated the same, or we could lower the threshold with mallopt.