Example for Mir-Random fails

[Brad]

I just do not know enough about the D libraries to figure out 
what is wrong.  I know it is a case of type mismatch.  The 
example appears on the Mir-Random page as listed under DUB:
https://code.dlang.org/packages/mir-random

The code looks like this:

```d
void main()
{
     import mir.random;
     import mir.random.variable: normalVar;
     import mir.random.algorithm: randomSlice;

     // Engines are allocated on stack or global
     auto rng = Random(unpredictableSeed);
     auto sample = rng.randomSlice(normalVar, 10);

     import std.stdio;
     sample[rng.randIndex($)].writeln;
}
```

This is the error I am seeing (I slapped the sample code into 
another program that flips a coin... It compiles file without the 
sample code):

```d
source\flipcoin.d(39,20): Error: constructor 
`std.random.MersenneTwisterEngine!(uint, 32LU, 624LU, 397LU, 
31LU, 2567483615u, 11LU, 4294967295u, 7LU, 2636928640u, 15LU, 
4022730752u, 18LU, 1812433253u).MersenneTwisterEngine.this(uint 
value)` is not callable using argument types `(ulong)`
source\flipcoin.d(39,20):        cannot pass argument 
`unpredictableSeed()` of type `ulong` to parameter `uint value`
```

Obviously it is a type mismatch - I have tried using to!uint to 
convert the result from unpredictableSeed to a type that will 
match - but that just causes more errors.

Thank you in advance.