Add 74LS393 for bank switching

[Gregory Bowne]

My goal is to start with 64k on board but extend the memory to 1MB via bank switching.

I think 1MB would be useful for most things like CP/M, etc.

The rest of the RAM will be on a 8 bit ISA card we will design.

A 74LS393 might be useful for this

I would like to use 4114s, 4116s, or 4164s fir this purpose.

Create a memory management system that uses this.