allocated_bytes_with_headers function

[stepancheg]

We are implementing memory profiler, and for such profiler we need to know allocation overhead as precise as possible (especially for small bumps where header size overhead might be significant).

allocated_bytes function does not include header size.

So add allocated_bytes_with_headers which returns the amount of memory requested from rust allocator.

[stepancheg]

Applied the suggestions.

[stepancheg]

No problem! Added quickcheck test.

[fitzgen]

Looks like MIRI tests are failing in CI. I suspect this is due to a change in MIRI and existing code here, but I don't have the time to investigate at the moment.

[fitzgen]

Thanks!