Get Klaus Dormann's test suite passing

[olefriis]

Trying to get these functional tests passing: https://github.com/Klaus2m5/6502_65C02_functional_tests

Currently, you have to manually copy bin_files/6502_functional_test.bin from that repo to the spec folder in this repo.

Running rake spec then produces a lot of output. Currently I am comparing this to the output of running the specs from https://github.com/hainesr/mos6502, where Mos6502::Cpu#inspect has been modified to

    def inspect
      format(
        'a: 0x%02x, x: 0x%02x, y: 0x%02x, sp: 0x%02x, ' \
        'pc: 0x%04x, op: 0x%02x, status: 0b%08b, memory: %i',
        @a, @x, @y, @sp, @pc, @memory.get(@pc), @status.encode, @memory.sum
      )
    end

...to reflect changes to the memory.

However, I think getting these functional tests passing requires us to implement BCD logic. I'm not sure I'm up for that. But let's see how far we can get.

This is a very nice (though somewhat brief) overview of all the MOS 6502 instructions: https://www.masswerk.at/6502/6502_instruction_set.html#SBC