Add SRAM to ATmega328P.
Add TODO.
Add ProgramMemory.

This replaces ATmege328P.@flash_memory, which was an instance of
NonVolatileMemory.

Persistence may be needed to ProgramMemory at a later date.

NonVolatileMemory did too much for now - it assumed that the memory
would have to be persisted to a file but we do not know that we will
actually need that capability at this early stage of development.