Bug: Fails to compile when size_t is not the same size as unsigned long long

[ZigaSajovic]

In the implementation size_t is used. But the implementation of flat_maprequires this type to be the same size as unsigned long long. But this is not true on all platforms (e.g. emscripten). Hence, when compiling one gets a hard-error for narrowing conversions from unsigned long long to <unsigned type of smaller size>.
The bug is easily fixed by defining

using ska::size_t = unsigned long long

And replacing the size_t with ska::size_t in the implementation.

Also note that when size_t is of size 4 one gets a warning for overflow in some expressions.