Use uniform_real_distribution to provide floats in C++

[agroce]

Better distribution options, and should be able to take a DeepState_Unsigned factory as a generator...

[agroce]

Probably useful to give users access to some way to provide DeepState values to the C++ standard distributions in any case; might be useful in various situations (though since the underlying fuzzer generators won't be uniform, you won't really get a normal, etc. of course if you're using feedback)