XorshiftEngine in final class implementation.
Small Ddoc, stylistic and unittest tweaks.
Test RNGs' opEquals for case where RNG types are not the same.
Mark @safe as much of each RNG is as possible.

The Linear Congruential and Xorshift engines are entirely @safe;
Mersenne Twister is @safe except for its range-based seed(), which
can be @safe or not depending on its input.