Add a generic PRNG with built-in entropy

[bshepherdson]

I keep trying to write games, and finding that I need a source of randomness.

It's possible to write a PRNG in software, and seed it from user input timing or the real-time functionality of the clock device, but that's a pain and not always practical.

This device, which will hopefully become standard issue, solves this problem by providing random unsigned numbers. The device contains an "internal entropy source", which would generally be implemented by using the emulator's host system's own RNG.

If reproducable numbers are important, software can seed the generator with a value of its own.