Made file_flags ops constexpr

async_io_dispatcher instances will now wait for any extant ops to complete before destructing. This solves a valgrind memory error caused by dangling op instances.

Unit tests should now print in colour on POSIX. Yay!
Tried to override utf-8 interpretation of input during coveralls upload
Cleaned up the async_io/errors unit test so it now works perfectly with no fuzz.
Hacked around MSVC's compile error with catch(...) in template's :(