Checking if an Integer is an Exact Binary Power

[Nordlöw via Digitalmars-d]

Wanted: CT-trait and run-time predicate for checking whether its 
single integer parameter is an exact power of two.

I guess

https://dlang.org/phobos/std_math.html#.truncPow2

or

https://dlang.org/phobos/std_math.html#.nextPow2

could be reused, but I bet there's a more efficient way of 
checking this.