Add a method to randomize the initial RAM

[zer0x64]

Multiple games uses the initial RAM content to seed their RNGs. We should allow to randomize the content of the RAM.

There are multiple ways to do it. We could add rand as a default feature and randomize the RAM when created if the feature is present, or add a randomize_ram() method on the emulator that takes a Rng as an argument. We need to make sure that this does not break portability however.