Add unittest for Mersenne Twisters with non-standard word size

The word size for `MersenneTwisterEngine` is determined by the template
parameter `w`, not `UIntType`.  For example, a generator with identical
parameters to the standard `Mt19937` but with a `ulong` wordtype should
not produce different results to the standard `uint`-based generator.

This patch adds unittests for the first and 10_000'th values of the
sequences generated by a variety of Mersenne Twister implementations
with non-standard template parameter values.

The values in this unittest can be validated by comparison to C++11 by
compiling and running the following C++ program:

/****************************************************************/

template<class UIntType, size_t w>
void mt_result ()
{
    std::mersenne_twister_engine<
        UIntType, w, 624, 397, 31,
        0x9908b0df, 11, 0xffffffff, 7,
        0x9d2c5680, 15,
        0xefc60000, 18, 1812433253> gen;

    gen.seed(std::mt19937::default_seed);
    std::cout << gen() << std::endl;
    for (int i = 0; i < 9998; ++i)
        gen();
    std::cout << gen() << std::endl;
    std::cout << std::endl;
}

int main ()
{
    mt_result<uint32_t, 32>();
    mt_result<uint64_t, 32>();
    mt_result<uint64_t, 48>();
    mt_result<uint64_t, 64>();
}
/****************************************************************/

Note that the `for` loop in this example advances the generator 9998
times compared to the D unittest's `popFrontN(9999)` because the first
`gen()` call already advances the generator once.

Author: WebDrake

std/random.d

@@ -956,8 +956,20 @@ alias Mt19937_64 = MersenneTwisterEngine!(ulong, 64, 312, 156, 31,
                                                         0x9d2c5680, 15,
                                                         0xefc60000, 18, 1812433253);
 
-    foreach (R; std.meta.AliasSeq!(MT!(uint, 32), MT!(ulong, 32), MT!(ulong, 48), MT!(ulong, 64)))
+    ulong[] expectedFirstValue = [3499211612uL, 3499211612uL,
+                                  171143175841277uL, 1145028863177033374uL];
+
+    ulong[] expected10kValue = [4123659995uL, 4123659995uL,
+                                51991688252792uL, 3031481165133029945uL];
+
+    foreach (i, R; std.meta.AliasSeq!(MT!(uint, 32), MT!(ulong, 32), MT!(ulong, 48), MT!(ulong, 64)))
+    {
         auto a = R();
+        a.seed(a.defaultSeed); // checks that some alternative paths in `seed` are utilized
+        assert(a.front == expectedFirstValue[i]);
+        a.popFrontN(9999);
+        assert(a.front == expected10kValue[i]);
+    }
 }