Address mapping - do it right

[ddrcode]

Some form of addreess mapping is currently implemented as a bunch of if conditions.
Let's abstract it properly.

Tasks

• Any adress-bus attached device can't be accessed directly. The only way to interact with should be via machine's get_byte, set_byte methods (perhaps worth renaming to read_byte / write_byte)
• read_word / write_word are just a convenience functions on machine level, not on memory-device
• unit tests!
• get rid of the current concept of Memory struct that has rom/rom fields. Separate them!
• the minimalistic implementation (MOS6502_Machine) - that cen be considered as implementation of Default trait, should consist of 64kB of RAM and 6-bytes ROM for reset/irq/nmi vectors. Don't hardcoded vector addresses anymore on machine's start function.
• for C64 implement at least a primitive version of PLA-82S100 - the memory "bus" logic
• on C64 level connect CIA, RAM, ROM and VIC_II via memory address bus and don't refer to them directly from C64 logic.
• Make the proper use of Byte 0, that may prevent writing access at some occasions
• (optional) would be great to have debugger being aware of the source of data

References:

• "Address decoding", by Wilson Mines
  That's rather overkill for this topic (explains address decoding on electronic level), but gives pretty good understanding how it works.
• "The C64 PLA Dissected", by Thomas ’skoe’ Giesel
  Full (I mean - full!) description of C64's PLA. On page 8 (section 2.6) there is good explanation of the logic - that looks like a great source of info for writing C64-specific tests