Sometimes, dealing with move-semantics in C++ can be challenging. To
mitigate this problem, this commit introduces `ClassSentinel`, a simple
class that can be added as a member of a class and that will allow the
user to easily monitor if an instance of a class is used after being
moved in an unwanted way. To do so, simply call the
`ClassSentinel::check` method on the class member.

`ClassSentinel` performs similar (but less reliable) checks on usage of
destroyed objects.

`ClassSentinel` can also (optionally, though the `SENTINEL_STACKTRACES`
macro) collect stack traces of the points where the object was moved or
destroyed.

This commit also includes some basic testing for the class.