Trigger dtors on unload of a shared library

[dtolnay]

Ctors run when I load a shared library, but its dtors do not run when that library is unloaded.

As you commented in dtolnay/inventory#1 (comment), fixing this may involve changing over from atexit to __cxa_atexit.

[mmastrac]

I spent a little while building up some test cases for this and I believe that this is working for all three platform and +crt_static & -crt_static:

Linux +crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426254#L519
Linux -crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426254#L552
OSX: +crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426257#L181
OSX: -crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426257#L214
Win: +crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426261#L142
Win: -crt_static: https://travis-ci.org/mmastrac/rust-ctor/jobs/473426261#L177

I'm creating a follow-up bug on this to automate some of the testing around this, but for now I think I can call this working. I'll release 0.1.5 with some doc fixes to indicate that this crate works for bin and cdylib based on these findings.

[dtolnay]

Nice, thanks! Confirmed that it works for me on Linux.