Implement general backend structure

[SergioLuis]

Developers: @sergioluis @mikelcaz Reviewers: @ugedo @jhvargas3112

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Implement a high-level backend (emulator) structure so the work on it can be divided. Bear in mind the following guidelines:

• Favor composition over inheritance.
• Rely on interfaces for composition!
• Split the code using packages.

Take this suggestion:

• The memory and the registers are simple concepts, but keeping them aside from the CPU should ease unit testing:
  • IMemory and IRegisters interfaces!
• The Clock needs to keep each instruction execution in sync with the necessary speed, but the unit tests don't need to run at the actual speed of the original system.
  • ÌClock interface!
• We should not need a complete CPU to test that the instructions actually work.
  • Static classes and methods to implement the instructions!
• The CPU needs the other components to work, but (probably) not the other way around.
  • Maybe we don't need a ICpu interface, but the CPU class will likely have a constructor that depends on IRegisters, IMemory and IClock.

[SergioLuis]

This depends on Implement general application structure.

[SergioLuis]

Package structure proposal:

• es.chipeit.lib.interfaces: interfaces for internal use only. The purpose of the interfaces is to reduce class coupling, making it easier to implement tests.
• es.chipeit.lib.io: interfaces and classes for other applications to implement input / output. For example, a class to issue key strokes to the emulator, or the interfaces or abstract classes to implement / extend by the application to receive graphics and sound commands.
• es.chipeit.lib.emulator: classes to interact with the emulation process. Load ROMs, load snapshots, start and stop the emulation, and connecting the I/O.
• es.chipeit.lib.core: the private part of the library. The software implementation of the hardware being emulated.

[SergioLuis]

Initial class and methods proposal by package:

interfaces:
   - ICpu
   - IMemory
   - IClock
   - IRegisters
   - ITimer

io:
   + ISoundPlayer
     + startPlaying() : void
     + stopPlaying() : void

   + IGraphicsRenderer
     + render(graphicMemory : byte[]) : void

   + KeyEnum : flags { 0, 1, 2, 3, 4, (...) E, F }

   + Keyboard
     + keyPressed(key : KeyEnum) : void
     + keyReleased(key : KeyEnum) : void

emulator:
   + Chipeit
     + keys : Keys (get)
     + Chipeit(soundPlayer : ISoundPlayer, gr : IGraphicsRenderer) : ctor
     + loadRom(rom : byte[]) : void
     + startEmulation() : void
     + stopEmulation() : void
     + getSnapshot() : SnapshotData
     + loadSnapshot(data : SnapshotData)

   + SnapshotData
     + <<static>> loadFrom(buffer : byte[]) : SnapshotData
     + writeTo(buffer : byte[]) : int

   + EmulationParameters
     + backgroundColor : int (get / set)
     + foregroundColor : int (get / set)
     + clockSpeed : short (get / set)
     + enableLog : boolean (get / set)

core:
   - Cpu : ICpu
   - Memory : IMemory
   - Clock : IClock
   - Registers : IRegisters
   - SoundTimer : ITimer
   - DelayTimer : ITimer
   - <<static>> InstructionSet

[SergioLuis]

PR #20 implements this.