Enhanced debugging

[nmcclatchey]

Improves debugging features of the library. Specifically,

• Adds recursion checking for all exclusive-mode operations in shared_mutex.
• Adds an explanatory message (instead of just calling std::terminate()) if NDEBUG is undefined and a thread is moved to or destroyed while in a joinable state.
• Makes certain cases of inheritance private to better conform to the standard.

[nmcclatchey]

I'm not really sure why GitHub Desktop added all the extraneous commits at the beginning.

A question: Should the recursive-lock checking be disabled by NDEBUG? The C++ standard only mandates "undefined behavior", so defaulting to high-performance would be the expected behavior. I recommend the following: Document a new macro: STDMUTEX_RECURSION_CHECKS.

• If explicitly set (to 1 or 0), follow user preference.
• If not set, set it to !defined(NDEBUG).

I've added a commit with this behavior, to be reverted if there is no support for it.