remove pessimizing moves on return

[weatherhead99]

remove several statements containing "return std::move(x)" which, according to the c++ standard 12.8/31 is pessimizing, because these objects are eligible for the NRVO. This issues a warning in gcc 9 otherwise.

[acki-m]

seems to be not working with the older compilers

[SnapperTT]

I'm getting this problem too. May it be fixed by a macro?

#ifdef __GNUC__
	#if __GNUC__ >= 9
		#define MOVE(X) X
	#else
		#define MOVE(X) std::move(X)
	#endif
#else
	#define MOVE(X) std::move(X)
#end

And if so, are you happy for me to submit PR?