Add unsafe test to check that dropped values are zeroized

[dvdplm]

Test that zeroization actually occurs when a boxed value goes out of scope.

Uses unsafe code, likely relies on UB and adds a custom allocator, hence the draft: is something like this acceptable?

c.f. iqlusioninc/crates#1310

[tarcieri]

You can use drop_in_place to invoke the drop handler (see other tests in the same module)

[dvdplm]

I saw those tests but felt that, given how magic drop_in_place is, readers would likely be mroe familiar with a plain old scope ending. Do you disagree?

[dvdplm]

I guess the original question still stands: is something like this acceptable? I'm on the fence myself, hence the draft.

[newpavlov]

Personally, while I am fine with having such test, I don't see much worth in it. I will leave the final decision to @tarcieri.

As a small suggestion, it may be worth to slightly modify how the test is implemented. Instead of reading leaked memory in the test itself, it may be better to allocate a known size (e.g. 1024 bytes) using SecretBox and then in dealloc check that deallocated memory with such size should be filled with zeros.