Take no exit

[Kixunil]

These changes add take_no_exit() function which uses sentinel value instead of exiting.

[Sgeo]

I'm not entirely sure what the point of release_sentinel is.

[Kixunil]

It's optional if someone wants to implement custom handling. (i.e. logging if drop was called instead of release_sentinel or conditionally panic anyway to cause abort). It being unsafe is just optimization. impl for Option<T> doesn't use it.

[Kixunil]

Hmm maybe I should've provided default impl...

[Kixunil]

OK, I've provided default impl. Option<T> still overrides it to optimize.