Investigate whether 'unused' memory areas can safely be ignored

[kendalharland]

We could forgo allocating 7 unused memory areas if this is the case. This would offer a slight speed up for our tests and slightly faster emulator startup.

The addresses would have to be mapped according to their true offsets to protect the rest of the emulator. e.g:

class MemoryMap {
    final ByteBuffer _buffer = new Uint8List(MEMORY_SIZE_WITHOUT_UNUSED_AREAS).buffer;
    ...
}

MemoryBlock internalWorkBlock(ByteBuffer b) => 
    new MemoryBlock(
        b,
        0x2000000 - SIZEOF_UNUSED_AREA_1,
        0x203ffff - SIZEOF_UNUSED_AREA_1
    ); 
    ... 

and so on. This may not be worth the trouble, or possible, but worth investigating sometime later.