At the moment CYGWIN pthreads implementation doesn't support the use of constexpr for static allocation versions. The way they define things results in a reinterpret_cast which violates constexpr.
- added read / write lock support
- added non-fatal error support to allow use of mutex in fatal error reporting pathway
- isolated pthread implementation to it own header/cpp file pair
- expanded unit tests to cover new code as well as better test existing mutex
- removed a layer of complexity that added no real value
