[stella] Random number generators

[jimn8@xxxxxxxxxx (Jim Nitchals)]

Communist Mutants did it like this:
RANDOM LDA RND
	ASL
	ASL
	EOR SHIPX
	ADC RND
	ADC FRAMEL
	STA RND
	RTS

It uses an ongoing seed (RND), and combines it with the ship's horizontal
position and the current video frame number to produce a new seed.  It's
simple but relies a bit on the randomness of the ship's X position to
yield good randomness in itself.

Suicide Mission uses a linear feedback shift register (LFSR).  Here's the
code:
RND    LDA RNDL
       LSR
       LSR
       SBC RNDL
       LSR
       ROR RNDH
       ROR RNDL
       LDA RNDL
       RTS

For simplicity of design, this approach is better but it requires 2 bytes
of RAM.  If the zero-page RAM is in short supply you could modify RND to
use Supercharger memory instead.